one of my friends had the same sort of situation, but he happened to be a pretty big dude and not someone to be fucked with, also had all sorts of hate for woman beaters, apparently they knocked some sense into the dude and he left town... thats not always waht will happen but when a guy like this gets wind of some abuse they will give it right back. im not sure exactly what you can do, because if you anonymously tip off the police or something, if he is indeed serious, he could kill her. dont even know what to tell you buddy, all you can do is wait next time it happens and actually physically take her to the police station, then theres nothing he can do to her because because she is in police custody.
good luck.