Point her here.  Lots of diverse advice from experienced people.

Other than being a supportive and non-judgemental shoulder to cry on?  I'm not sure there is much you can specifically do or say.  

I would encourage her to take care of herself.. so she is best equipped to deal with the stress.

Let her know that whatever her feelings are, they are ok. A lot of stepmoms feel guilty for having any negative feelings about their situation. Let her know that setting boundaries is ok, as long as she treats the kids with kindness and respect. 

This was me in 2019/2020. DH took BM to court for more visitation and joint legal custody, which should have been straight forward, but definitely wasn't. The court battle turned nasty thanks to BM and she pulled false allegations, emergency injunctions to stop parenting time, drug my name through the mud, and starting attacking random family members (most of whom she had never met) to add an extra layer of shit. It was madness, all for BM and GF to keep their firm control over SS and DH and keep their CS payments maximized. BM threw everything and the kitchen sink to argue why DH shouldn't have more than 6 weeks per year and NO legal custody. 

I was in graduate school at the time, to become a therapist, and in one of my therapy classes, where we tested different therapeutic methods on each other, I broke down crying. The weight of the world was too much between school, work, the custody battle's strain on my new marriage, and dealing with the BM/GF/SS scenario. I had severe anxiety and almost PTSD from being drug into the custody battle and having every aspect of my life and our families lives under fire. Most of our friends are couples and the men were friends with DH in school, so they know of BM. I felt I couldn't really talk to them or their wives about it. I couldn't really talk to my friends because they aren't SMs or children of divorce, so they would just play the "why can't you all just get along?" card. So I found steptalk which finally gave me a sounding board for people who get it. I've been here daily since. We inevitably ended up settling after the judge said he wouldn't order in our favor if we took it to trial and basically outlined what his ruling would be (which was exactly what we already had). That is where we kind of dropped rope and gave up caring about the outcomes for SS. 

If it were me, I would want a supportive ear. I would want someone I could vent to who gets it and doesn't get irritated hearing about it; someone who won't pull the "just get along" card. Sometimes the only way to process and heal is by talking it out and processing it outloud. 

Protecting my SKid, my DW, and myself from the shallow and polluted end of the Skid;s gene pool was my focus and the focus my DW and I had during those days.

From what you describe, the odds of a change in custody are slim. Just make sure that your friend and her DH have every spec of dirt available on BM and her felon F-buddy DH and scrub her with it all in court.

Not an easy experience, as many STalkers know full well.

Be there to listen. This stuff is so out there crazy that I have no-one in my real life that I can unburden all  this sh*t to. I don't want my family to hear all the ups and downs of the drama. I can't tell our mutal friends because that would just make everything awkward and my friends just wouldn't get it. So I come here instead. But I'd love a real life friend who could really listen and understand and not judge and not think that they have all the answers when they don't have a clue.