I am concerned about your friend's emotional well being. She has been a victim of domestic violence, and I understand that she continues to feel unsafe and victimized.
There is also lots of evidence that the risk to physical safety increases during separations or when a partner leaves the relationship. You are all worried about that given your boyfriend is with her...and for good reason.
Some would say,"she's an adult and can make her own mistakes" or " who am I to tell her that sex with your support base might not be easy therapy without other unwanted consequences." but she is vulnerable and surviving trauma.
Sounds like what she needs is time to heal, learn to feel feel safe emotionally and physically. Fucking her is weird therapy, even if that is what she thinks she wants. You doubt her self awareness with good reason. She is someone who has been traumatized and is a sensitive space and unlikely to have full clarity. I think that offering emotional and practical support would be best right now. Maybe keeping some clear boundaries, and recommending professional counseling support would be best.
Separating, let alone in the context of fear and violence, is enough to deal with. Sex with friends, poor communication, trying to feel better after a shitty relationship..all sounds like a recipe for a lot of heartache.
Sounds like you both care deeply for her safety and well being and for your friendship. Best of lucky navigating this situation.