As much as you want to help your friend, unfortunately she can only help herself. As harsh as that sounds its the truth. I think there is alot more to this relationship that she has probably told you. Violence is a form of weakness, any man that will raise a hand to a woman clearly have issues. You may find that he has been controlling her through manipulation, if this is the case getting yourself too involved you will only end up putting her in a very awkward position, and 9 times out of 10 a woman in this situation will normally stands by the man. The best advice i can give you is to listen to her, never judge or tell her what to do ( as much as you want to) it may take a couple of weeks, or a couple of months + but one day she will finally come to her senses.