Nobody answered your question because it's a very difficult question to answer.
I don't ever question my girlfriends if they hang out with another guy. Why? Because I believe I'm the best guy they'll ever get. I also trust them otherwise I wouldn't be with them.
The jealous boyfriend is the worst thing you can be. It will guarantee that she will cheat on you. I've seen the best, most honest women cheat on their boyfriends because their boyfriend was the jealous type. There's two types of women (broadly speaking). One type will cheat because that's just the person she is. The other type is actually loyal, but she doesn't have the balls to end a relationship when she becomes unhappy. Therefor she cheats because she's no longer attracted to the guy and she wants out. Most women fall into the second category.
It's usually pretty easy to spot a girl in the first category if you have experience with them. Women in the second category could be anyone. I know women who never thought they would cheat, and nobody thought they would, yet they did because they were unhappy.
In your situation you need to trust your girlfriend. You shouldn't be doubting her just yet. If you notice more things later then you can deal with it. If you bring it up now she will just get defensive and want to see the guy more.
One thing about women that I learned from David X that works like a charm is this "If you tell a girl not to do something she will". Basically whatever you tell her, she will do the opposite. I did this with my last girlfriend all the time. I would tell her "You should hook up with so and so, he's cute". Every time I told her to go hook up with other guys she would become more and more faithful.
This is why it's so hard to answer your question. On one hand you don't want to look like you're controlling and jealous, on the other hand you don't feel comfortable with your girlfriend hanging out with this guy. You should wait and see what happens. If you need to address it down the road honesty is always the best. "I'm not comfortable with you hanging out with this guy. I feel like you change when you're around him and I don't like that." You need to be direct and to the point but don't accuse her of anything. You'll also want to be stern but non-emotional. Hear her out and if you don't like what you hear walk away.
The only thing you would do by bringing it up now is look like the jealous type and drive her into this guys arms even more.