Gfs guy friend trying to make a move?

Me and my girlfriend are really happy together and we love each other a lot. We also both trust each other. But recently it's become a little obvious that one of her guy friends may be trying to make a move on her.

I don't know much about this guy, but I do know that he and my girlfriend used to be very close. They drifted apart for about a year but he's been trying to get into contact with her. I noticed after he posted some seriously flirty messages on her Facebook wall a few weeks ago. Now I won't go into detail about what they said, but I showed them to some close friends and they all think it's obvious he's hitting on her.

Here's the catch. I'm not sure if my girlfriend realized this, but it almost seems like she was flirting back. And he also asked her to meet him soon so they could catch up... and she agreed. So my question is, am I wrong to be uncomfortable with this? I mean it's so obvious he's hitting on her, so why would she agree to meet him? And how should I talk to my girlfriend about this without coming off as controlling?
Gfs guy friend trying to make a move?
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