What would you do?

First, I have to say that this is common behavior for the victim in an abusive relationship. Getting out is very hard and in the abusers are often very manipulating. Second, there is one big weapon you have against this man. Her daughter. The best chance you have of getting her out of this relationship is showing her how bad it is for her daughter. You really need to focus on the daughter's wellfare and push that point. Even if her daughter is not directly affected, she is seeing how her mother is being treated. Third, you may not be able to get her away as she is the only one who can make that decision. But you can try and provide support when she does need it. It is frustrating, but do not get upset and walk away. Isolation is one of the abusers best weapons. If anything tell her that you still want to spend time with her, but until she decides to stand up for herself you can not help her with that situation any further. Finally, If you can not convince her to leave, do the next best thing and be strong for her where she is not. Protect her child. Do what you can to make sure that whatever mistakes she is making do not hurt her daughter. For example, offer to spend time with the daughter more. Maybe even take her out every now and then. Who knows maybe this could also lead to getting the mother out more and away from the abuser.