Hey Benjiman. It's nice that you feel you want to ensure your girlfriend's safety, but n this instance it certainly didn't seem to need "handling". You say he was drunk and walked off-could you just say to your girlfriend, "How rude!" Or wait 'til she says something, and say something like, "Well you can't blame him, you are beautiful :) ". If he had become lairy, I am sure your girlfriend would have appreciated it if you told him quietly but with deadly seriousness that you had let the touching go, but does he really want to push his luck?
In all fairness, this kind of thing would perhaps happen if you weren't with your g/f, and she was with friends. I am sure she handles it then too.
I would be happy for my partner just to tell me I have a very feelable ar*e, and that be it! It is nice to feel safe, but only in situations that warrant it: reacting with the same level of outrage at every minor thing can seem possessive.
Sorry it's a bit long winded, hope that helps!