I think it's best that I don't try to fight the guy who groped my gf while she was drunk.
Have you tried to keep your girlfriend from drinking too much in the past?
Yes i have tried my best to keep her sober, but I can't always be with her.
I see that you're frustrated with your girlfriend's drinking, but it's important that you have her back in this situation. Being unwillingly groped is a form of assault, and you should protect her in that kind of situation.
I'm not going to fight him, because he's bigger than me. I might report him to the police though.
That might be a good idea as well. It's hard to tell if this is a routine thing for him. It could stop him from victimizing other people.