Get the book "Stop Walking On Eggshells" it will give you insight into BDP and strategies for setting boundaries and dealing with a person who has this disorder. Be aware that people with this disorder tend to become worse over time not better. 

My brother's wife is a child of divorce and is basically the grown-up version of everybody's horrible stepdaughter. She very much uses her diagnosis as an excuse for her bad behavior and to control the people around her and get her way. She is highly manipulative and uses the threat of harming herself or throwing a tantrum in order get what she wants. People like this are set off by the slightest hint of an interpersonal conflict. They just can't handle conflicts like a normal person. Any small issue turns into a full-blown crisis. They are unpredictable and the people around them learn to handle them with kid gloves as to try to not set them off.

If she has been out of your DHs life for some time now the best course of action is to keep her at arm's length. 

I believe that the BM in my life and both her daughters - SD25 and SD24 have personality disorders, although not formally diagnosed up until now.    My one suggestion would be that although yes, it is a recognised condition - it should NOT be used as an excuse for treating family and friends like shit.  I have poor mental health at times (have been hospitalised in the past) but have never felt that this allows me to treat others poorly.  If someone has a mental health condition or personality disorder, they need firm boundaries from others, not carte blanche to act like a mini dictator.   

So I would say that you and your DH should expect her to treat you with respect, and your DH should not allow himself to be treated as a "walking wallet".  It is not acceptable for SD to only approach him if she wants money.  Some people use the diagnosis of conditions like this as an excuse to sit back and be lazy all their lives - be careful your DH does not collude or enable this. 

I would definitely start reading up on it so you can develop tools to deal with her behavior. People with BPD can be exhausting and emotionally draining if you do not recognize Thier attempts at manipulation and develop healthy boundaries to protect yourself.

They use a lot of smoke and mirrors and gaslighting to confuse you.

BPD is a very challenging issue to deal with, both for the person with it, and family/friends. Do read up on how to set firm boundaries. Some people with BPD do have insight and want to feel better, and there are some therapies that can help (as well as meds for the issues that go along with it, like anxiety and depression). This type of person tends to internalize feelings and harm themselves instead of acting out against others. But many do NOT have insight and are unable to see how their behavior causes all the bad things that happen to them - they feel like eternal victims and take it out on everyone around them. It can lead to destruction in all areas of their lives.

Boundaries are really, really important. You can support her without enabling her.

BM of SD10 has admitted to having BPD. Over the years, her behavior patterns have become more predictable, so we know what to expect. But it doesn't change. There is no medication or "cure" for BPD. She will go in waves of having "moments of clarity" and claiming to better herself, and having a realization of "what she has done" to people around her. This is usually when she has a new boyfriend or needs something from DH and I. She manipulates situations. When she is not having one of these moments (usually last for a few months), she will go to the other extreme. Threatening court, guilty of PAS, verbally attacking DH and myself, suspected drug use, the list goes on. BM also tries to excuse her bad behavior by claiming she gets "triggered" and just can't help but "go crazy" when it comes to her kids because she is an "over protective mother". 

SD is only 10, but she has already been diagnosed with anxiety, depression, ADHD and, ODD. Doctors typically don't diagnose BPD until 18 years of age. I would not be surprised at all if SD has this disorder as well when she's an adult. She already displays some of the same symptoms. Blaming others for actions, not taking responsibility, manipulating situations, using relationships with others for personal gain, "me against the world" mentality, constantly being the victim, and even paranoia that everyone is working "against" her, etc. 

It's a very complex disorder. I would agree with reading up on it to gain some insight. 

I strongly suspect my brother-in-law to be BPD (he sees a therapist but my sister has not said what specifically for).  His behavior (sudden mood swings and outbursts) certainly fits the pattern well.  

As others have said, this is really a life-long struggle.  Personality disorders can't be treated well with medication alone as it is not really a chemical imbalance but rather a fundamentally flawed development that leads up to it.  It requires intensive therapy for the BPD person to cope with others would perceive as minor annoyances in life, and to communicate their wants and needs effectively to maintain relationships.  It is essentially re-building and maintaining basic life skills.  If her pattern up unitl know has been to push her father away, then blame him - that will likely repeat.

From your perspective, you can offer comfort and a sympathetic ear to your partner, but maintain distance from the child.  You mentioned she is in college - how are her grades and what are her plans post-graduation (which is likely to be soon given she is 21, I would think).  I would be VERY concerned if your SO is willing to let her return home to live with him after graduation.

The good news is she is clearly seeking treatment and now has a diagnosis to work with.  Also a postiive note that she is able to handle school, so she may be functional enough to live independently.

Good luck with your research.  I'm sure there are also support groups for families of NPD people - you and your SO can maybe explore those together.

Not in its entirety yet, but I believe it does describe both my DH and his daughter that is so difficult.  I always thought his dramatic mood swings were a form of bipolar disorder, but my sister (who is a licensed therapist) suggested to me that his issues sound more like BPD.

People often talk about establishing boundaries on this message board but then don't give the instructions of how to do that.  This book does go into more detail about the 'how to' of setting boundaries.  

For example, When DH is unable to handle a pressure topic, I've learned to say "we can discuss it further when you've had a chance to calm down".   
 

Another boundary is having my own relaxation room, just like it's important that DH has his own man cave in the basement where he can regroup and collect his composure.  
 

Another symptom of BPD is impulsivity and recklessness.  Whenever DH was driving the car, I would literally recline my passenger seat all the way down, and close my eyes the entire time, to keep the peace and keep from screaming in fear.  After he crashed the car, we worked out in counseling that I would always be the driver of the car.  That was another boundary, he doesn't get to drive when we go someplace together.   

I described their behavior to the pastor, that we also enlisted to counsel us, that DH has these dramatic mood swings and meltdowns whenever conflict arises.  He can't handle the garden variety of challenges and deflects them to me to handle.  Or blames me for the actions of other people.  That's why I always choose my words carefully and the timing to bring up a hot topic.  

DH is usually embarrassed afterwards that he over-reacted to something and I've learned to avoid the triggers of those hot topics.  One of the biggest hot topics is his daughter, so I avoid any discussion of her like the plague.  
 

It's characterized by black and white thinking - so someone is all good or all bad; no sense of self, so will often reinvent themselves in sometimes extreme ways; attention-seeking and risk-taking behavior (promiscuity, addiction, gambling, excessive spending, eating disorders, suicidal threats or attempts); extreme fear of abandonment - which often manifests in pushing people away. It also seems to lead to a martyr or victim complex - no one appreciates them and everyone is out to get them; histrionic reactions and emotional dysregulation; and a complete lack of respect for anyone's boundaries, in fact, putting up boundaries is often seen as a form of abandonment by a person with BPD. I've also read that for individuals with BPD "feelings are facts", so they can very convincingly share things a person has supposedly done against them when those things never happened. There are a lot of false claims of abuse. The theory is that individuals with BPD developed the disorder after extreme abuse, but I don't think that's always the case. The BM in my life is undiagnosed, but has "borderline tendencies" according to several counselors I've seen to deal with her behavior. I think - based on what DH has told me - that her mother was cold and likely said not nice things to her, but not extremely abusive. I've read that a borderline parent "can't exist without creating a borderline child", so maybe her mother was also borderline. BM is definitely fashioning SS into her borderline child. She is more emotionally close to him than her current DH. He's her confidante, her protector, and he has the same overblown, histrionic reactions that she does. I think parental alienation is very common from borderline (or narc) parents who will swear that what they're saying is 100% true and they are only trying to "protect" their child from the other parent. With that in mind, does BM also show borderline tendencies?

"I would be VERY concerned if your SO is willing to let her return home to live with him after graduation." This was my first thought as I was reading your post, OP. Your DH's daughter is going to be problematic for the rest of her life, so it behooves you to play the long game and get some firm boundaries in place. BPDs cause havoc wherever they go, so DO NOT allow this woman to live with you, EVER.

My DH's youngest daughter has BPD, and we have been estranged from her (DH's decision) for many years because of it. She has contacted us several times, only when she was in difficulty and had run out of other people to use. His daughter is in her mid thirties now, and has had hurt a lot of people over the years. She is by far the best liar I have encountered. Each time his daughter has contacted him, DH makes it clear that reunification can only occur in a therapeutic environment, and only if his daughter is ready to take responsibility for the things she did and the pain she has caused. That's always enough to make her disappear again.

You can have a peaceful life, OR you can have one with your SD. It's just that simple.

My DD38 has BPD. It's wrecked her life, and has at times come close to wrecking mine. I hope for your SD's sake she has excellent psychiatric support and care--she will need them intermittently throughout her life, though I understand the condition tempers somewhat as the individual gets older. I hope for your sake and for your partner's sake that you have the ability to set and maintain very firm boundaries, because with a BPD in your life, you're going to need it. And you are absolutely right--if your partner appears to be minimizing this situation, you need to get out now. True BPD is very serious, and you don't need the grief.

Some cases are high functioning and can be treated with therapy that involves managing the triggers.    Keeping stress at bay.    Not all BPDs. are skid row alcoholics and they do function to the outside observer.  They don't have these obvious addiction vices but the spouse can normally see the signs.  
 

I go into a store and come out with an extra candy bar.  DH goes into a repair shop and while  waiting he trades it in and comes out with a $90,000 motor home.    Reckless and impulsive spending is a symptom.    
 

i would recommend a book called "The Peacemaker" by Ken Sande.   
 

Im glad this came up. Munchkin SD14 last night said "my mother is mentally ill, shes mental".

So I started thinking more about this, only in an effort to help Munchkin.

I asked her today, and shes not at the point she wants to talk about it. So Im researching and learning, for when she is ready or we can afford therapy/a therapist.