A guy? his wife/girlfriend/woman is beating him up? Easy advice KICK HER ASS (just kidding simon just kiddding... maybe)
Unfortunaly it all comes down to her and it sounds like she is going to get herself killed... sad but true and if you did manage to get her away from this guy the next guy she finds will be the same guy with a different face!
I can't advocate anything illegal the easiest thing is if she would just agree to leave.
If this guy is crazy violent then let her come to your house. If he starts to be threatening get a court order stating that he is threatening you as well. Then when he comes up and knocks on the door give him both barrels of double aught straight though the door don't even open it just look out make sure he is centered and pull the trigger... you're safe because with the threatening injunction it's self defense!
This is the reality of our world their are two types humans and vermin. People who go around cutting peoples heads off or beating their woman (or men) to death are vermin. Vermin deserve what they get. Thats reality it sucks but thats the way it is.
You could tell the police on your own tell them the situation and sometimes if you tell the right policeman they are willing to go and "talk" to him.
Hopefully you will read this before simon deletes it...