I've learned that relationship drama is best to be left between the people involved. There has to be a reason he went crazy on her in the first place. Let it simmer down a day or 2, and just check on your friend and be like "dude you ok?" .. eventually, being his best friend, he'll eventually want to talk to you about what happened. While its not cool that he was physical with his GF in a not-good way, getting involved while the situation is still hot will just get you stuck in something you dont want to be in.

Give it time and he'll spill the details. If you're closer to him than his gf, you should avoid contacting the girl, or you'll betray his trust and damage your friendship. Women are very evil, vile creatures and should never be trusted. =)

edit - and about the guy saying to avoid alcohol, obviously has little to nothing to do with the situation. There was an ongoing instance in the relationship. I've seen sober people act far worse and hospitalize people. Its all in the personality, regardless of being sober or not.

Poor girl.  Having to put up with an angry, violent drunk like that.  If my best friend of twenty years pulled that shit on his girl, I'd disown the guy.  There's no excuse for that.