Dark has some good advice, as far as the alcohol goes, I can help with that (many times of being drunk on this kid's end).
Usually when alcohol is involved, this kind of crap happens. Especially this kind of drama, it's not rare, believe me. When most girls get drunk, they usually... Have... Fun... I guess you could say... More they're drunk and don't know what the hell they're doing and don't listen to rational thought (same goes for guys).
Now, best case scenario, you have this girl talk to the guy. Have her tell him that you didn't know she was taken, and that alcohol was involved. If you were slightly intoxicated this would help your situation, but I won't make assumptions on that part. If so, however, he needs to know that. If you were intoxicated to any point, and the guy listens to rational thought he'll think Ugh, so they were both drunk? Ok, well, this sucks. Drunk people are dumb, and make mistakes. Oh effing well. Or something like that.
Worst case scenario, he flogs your molly... (band humor... Hehe) After that, it's over. You can't talk to him, he won't talk to you. End of story.