I joined BPD family for help with my d16 (I've been away for awhile, we are in the eye of the storm for the moment)

and now could really use some advice for helping my sd17. Her mother has many BPD symptoms (which led to the divorce, she ran off with her best friend's husband and blames my dh for... .pretty much everything). But on the whole, she's been a caring, attentive mom. Her biggest problem is that she takes conflict with her children on as personal rejection, she makes them responsible for her feelings.
Lately her youngest, sd17, has been in conflict with her. This is a kid who has always been "the easy one" and has been really close to her mother. Now sd17 is staying with us, and saying she doesn't want to see her mom. She has a list of accusations against her mother (typical teen exaggerations). She is in counseling, and she's gone to another counselor with her mom for mediation.
After the most recent dust-up she became furious at dh because he didn't jump on the mom-is-the-worst bandwagon. She told a school counselor she was suicidal, and sd17 and dh spent the day at the ER for a suicide eval.
So here is my question: Any suggestions for how dh can validate sd's feelings without taking sides against mom? He does want to support mom and her time with sd, and honor the parenting plan. She is super sensitive on this topic. Ok, she's super sensitive on any personal topic. (I just realized that "she" is ambiguous. I meant sd is super sensitive, but it applies to mom as well.)