What was the nature of the assault? I mean how serious, did he grab her ass? or was it more extreme?

I'm sorry to hear that this happened to your gf, there are bad guys everywhere and I don't think you can blame the club, city or country. This kind of shit is universal unfortunately.

I would suggest, if your gf is up to it, returning to the club and try to spot this guy, either involve the police, or sort it out yourself.