I’ve been in such a situation - your friend could have been me 5 or 6 years ago. To be honest, I wasn’t prepared to listen to my friends if they tried to tell me that I shouldn’t be in the relationship, or that he was abusive. Having said that, one of the things I found most painful when I finally got away from him was the feeling of “you knew what was going on and did nothing” response I had to some of my friends. I realise now that they were in a horribly difficult position - not knowing whether to say anything or not.
For my money, the best bet is to tell your friend your concerns. And when she rejects them, back off (not from your concerns, but just don’t mention them again). Be supportive of her, even when she makes the wrong choices (which she probably will). Tell her how she deserves to be treated (she’ll probably try to tell you that her bf treats her like that already, even if he doesn’t). And be there for her when it all comes crashing in, because sooner or later it will. The hardest part may be that she tries to cut you off (either because she feels threatened by you telling her what she already knows about the guy, or (as in my case) the guy works to isolate her from her support networks of friends), but you need to make sure that she knows that you are there to help her, any time night or day.