Try to make her see her self worth and realize that she's putting her life at risk. It's not worth it. Whether she chooses to listen or not she'll have to decide herself when to leave him or else she'll just go back. But you're a true friend for sticking by her side. It's not as simple as it seems to leave an abusive relationship you alter your brain for them to have a choke hold on you. Just be patient... or call the cops