@Dedra12,
I think you realize you are enabling her. You are giving her a place to go, and someone to vent to, who isn't a therapist, a cop, or her mother.
I understand you want to be supportive and I imagine you can see there are now going to be two innocent children mixed up in this catastrophe.
If you provide an ultimatum, you may make things a lot worse for her and/or never see her again. But I can understand your not wanting to be used anymore.
Maybe the thing to do is to start leaning, but gently, on her. Let's call her Amy.
- "Amy, you can't stay here tonight; I've got company."
- "Amy, why don't you come over tomorrow and bring the baby and we'll have a nice girls' night and watch a chick flick?"
- "Amy, now that we're in the Emergency Room, I feel you need to tell the whole truth to the doctor. I will be with you to support you."
- "Amy, have you talked to your mom recently? Please remember her to me. I've always felt she's a really understanding person."
I'm not saying these are perfect solutions but the status quo has got to change. All of these statements gently challenge the status quo. The first one gives her a little pain, yes, but it might make her think twice before she accepts his crap again. The second offers support and an escape. The third gets an impartial third party involved in order to start making a record. And the fourth is an attempt to give her a nudge to involving her parents, who will most likely also want her to stop taking it.
I went to High School with a gal who took it for years. She finally felt empowered enough to take her son (who was not her abuser's child) and leave. Whenever I see her on social media (she lives in another state), I tell her how strong and brave she is.
Right now, your friend is scared. She is probably hearing from him about how horrible a person she is. She needs someone to tell her otherwise. It's a way to get her to stop believing that.