[EllaC]

my best friend was going through the same thing. he was such a nutter he chose to believe that her and I were an item behind his back. when it got to that point and my family was at stake I took a step back.
I tried and tired to get her to leave but "at the end of the day" she didn't leave until things hit rock bottom and one can only hope that this is rock bottom for your friend.
if you knew her better id say get hold of her family! if her family knew they would intervene in a way that might make her see sence. alternatively her friends. I tried to contact the authorities with my friend but they said outright "there's nothing you can do, if she doesn't want to leave him then nobody can force her too"
the best you can do is try to get her know her on a more personal level, try and milk as much info out of her about her personal life as you can and try intervene from there.

having been there and done that I can tell you if she doesn't want to leave then all of your efforts might be in vain. I worried myself stupid about my friend, it seemed none of her friends or family cared that he was beating her and it all go me nowhere! she didn't leave because "she still loved him" etc etc. and it took many months and many incidents before she left - when she was good and ready..