<human>: L.O.R. 3 years
Age m(20) f(21)

So yesterday me and my gf are at my friends dorm talking to him an a couple of his floor mates. And as we're leaving one of them tells her "by the way your very beautiful". We're already in the elevator so I didn't even hear/see who it was. 
She got all flustered like "oh my god did you hear what he said? He said I was beautiful. What an ego boost". This kind of upset me because I tell her she's beautiful all the time and she hardly cares. 
So today were supposed to be going to a free concert and he will be there. Should I be upset/jealous about this? My first instinct if he says something like that again is to punch his lights out. But I know that's just my testosterone speaking not my rationality. How should I handle this?

Condense the text into a few key points
<bot>: guy tells my girlfriend she's beautiful she's all giddy about it. We are going to be hanging out with him again tomorrow.