Generally speaking, I dont worry about guys flirting with the girls I date... especially if she's my girl friend. More then anything I find it ammusing, it's not like they have a prayer of "taking her away" from me... thats the mindset I have at least
After he left it would depend on my mood but I would probably joke about it in someway... like "I think he likes you

" then maybe say something about a fight to the death for my ladies honor while I brandish my butterknife... or grab her face and lick her as I claim to be marking my territory.
Or maybe doing something that an X girlfriend of mine used to do... she would always point it out when a girl was checking me out... it would go something like this:
Her: "That girl over there is staring at you babe"
Me: "oh yeah? well I am a very sexy man, I cant really blame her :-p"
Her: "To bad your mine!"
Her: "I'm gunna make her sooo jealous"
Then she would maul me like a tiger lol
that worked gloriously on me when she would do that... perhaps some modification of that would have the same effect
