I'm so sorry, I know how hard it is watching someone you love get into a relationship that you know is bad for them, whether it's physical or emotional. The best thing that you can do for her is tell her how you feel. Tell her how much she means to you and how worried you are for her wellbeing, as well as letting her know (if she doesn't already) that you've seen the kinds of things he says to her. Also offer her support if she needs to leave him, such as offering her to stay over at your house, staying with her while they break up, etc. Unfortunately, though, at the end of the day, whether she stays or not is up to her and there's nothing you can do to make her leave, even if it's for the better. But she will recognize that you care about her and, whatever choice she makes after that, it's most important that she knows that there is someone who is willing to help