How would you act towards this guy? He kissed my g/f.

So my g/f went out drinking with some friends and this guy that she used to work with kissed her….THREE TIMES! After she told me this happened I asked her a few dozen questions about the scenario, most of all I wanted to know: Why didn’t you say anything to stop it!?! ...my g/f is a completely non-confrontational person and she explained that he just ‘went in’ real fast as they were talking and she pulled away each time. She didn’t say anything b/c she didn’t want to make him feel bad or create an uncomfortable situation. I met this guy only once, no real conversation just handshake talk…“so you’re amy’s boyfriend eh?” ..“yep nice to meet you”...“nice to meet you too”...sort of thing. I’m bound to see this guy again when we go out, part of me wants to punch him in the face (what kind of jerk are you to kiss a girl who you know has a b/f? really…) but I can restrain my caveman instincts. I won’t resort to physical violence but i’m def. not going to be friendly. Is it a bad idea to confront him about it? Should I just try to ignore him?