I would tell her the following.

Over the course of the past x years you have not allowed me to help "parent/guide" SD in any way, I was dismissed and belittled. Now that you realize that you messed up with making the choice to not even co-parent with her father you want help. I can not and will not put myself in the position of being a scapegoat for you or your daughter. If you want to make an effort to better your childs life then you need to build communication with her father in a healthily and respectful way. I will always support my HUSBAND and help HIM with parenting when he communicates with me what is being done. But as far as working directly with you it will not happen. I am not her bio-parent and over the course of x years both you and her have made that clear. Would you willingly stick your hand in a rattle snakes cage, I think not.

Man, I would LOVE it if BM would acknowledge anything! I think you should call her back and thank her for her call - be very noncommittal to anything she asks for unless you feel like doing it. Just let her know that you'll talk about it with DH. Most likely she realizes she is in over her head with him and now wants to act like you are all one big happy extended family to spread the blame/responsibility around. Again, I would call back since she acknowledged so much and it is in your best interest to get the connection started with her. Her next step if you don't will be to contact DH with the same stuff, but this time she'll try to disclude you since you didn't respond so you "obviously don't care". Who knows what bullshit she'll get him to agree to and you'll still be subjected to drama through him with no control over your level of involvement. Keep a leash on her and tell DH that you'll "rise above past issues and do whats best for SS". Then you win points with him too, but you only need to get the both of you involved to the extent you want to;)

Can u just let dh deal with her? I would never trust bm. Even if she were sincere she wouldn't be able to play nice for long.

Hmmm....well, only YOU know YOUR BM best. If it were OUR BM calling, the only way I would respond is if she said her house was on fire and SD was in it. And THEN I would only respond to say, "Call 911, dumbass!!"

If you and DH decide to help Bm. I would come at SD as a united front. All of you, DH and BM and your self need to sit down together and make rules and consequences for SD bad behavior, talking back, not doing chores, bad grades. Whatever she is doing that needs to be addressed. Then SD needs to be sat down with all ya'll there and explained to that as parents that ya'll are now a united front and that Sd is to follow the rules at both house holds. That as parents ( and the SM should be included) ya'll will stay in contact with each other and that SD is no longer going to be able to play you as parents against each other to get her way.
If for any reason Bm doesn't want to do it that way. I would not offer to help and just keep doing what your doing. I would not offer to let SD come live with you and DH again and clean up the mess BM has made by letting SD have her way for the pass two years.
Any decision that you and DH make. I would let him tell Bm. You don't want Bm twisting things to be blamed on you in the future. They need to be DH and BM ideas and you just need to help enforce them. Then you can always say those were your ideas not mine I just help because you asked me too.