This is something I know a
great deal about, from experience. Lessons learned the hard way. Learn from my mistakes so that you do not make them yourself. That is what all us old folk hope for, that the life-illiterate younglings among us learn a thing or two from us.
So, here's the scoop. The issue, at its core, is one of trust. Not saying you don't trust her, not at all. I'm saying you don't trust
him. Nor should you. I assume you don't know him. You don't hang out with him. If you did it'd be less of an issue, right? Note on that later. What you need to do is communicate with your girlfriend instead of us on the forums. Forums are a bad place for relationship advice. If we had successful relationships, we'd be somewhere else, presumably with our boyfriends and girlfriends having copious amounts of unprotected sex.
But yes, talk to your girlfriend, express your concern. The way to do this, and this is important, is to have
her ask what's bothering
you. Because if you bring it up on your own then you're just crazy and jealous. So just clam up, be distracted or whatever and she'll eventually ask. If she doesn't, there's a red flag right there and I'll let you draw your own conclusions and solutions.
Other options. I had a "friendcrush" some time ago. I wanted a completely platonic relationship with a girl. I got it too, after a long and somewhat awkward period of "friendcourting". The friendship is great, but it painted her boyfriend's face all shades of jealous. So, solution, become his best friend. Now we're a threesome. It's a party now. He trusts me to not do anything inappropriate because he knows me, he trusts me, we're friends. What I'm suggesting is simple, perhaps you can befriend this guy too. If he's not open to your invitation, the olive branch, the bridge of friendship that you want to build, he probably just wants to lay your girlfriend and is bad news for you.
Option B, the nuclear option. I was with a girl ages ago, and I was super jealous of her hanging out with a friend. They hadn't been together, but I didn't know him, I didn't like
the way he looked at her, I didn't like the way he dressed, the way he talked, the way he walked,
I didn't like him. I feigned indifference, and I am amazing at feigning, but it bothered me terribly. She came to me and asked if I was the sort to get upset if she hung out with other guys, as we were seven days into our relationship, we were still
getting a feel for each other and she didn't want to chase me off. "No it's cool" I said. That afternoon I dove into a relationship with her best friend, and the following morning I ended the relationship I had with her. How this played out over the long run, my then ex-girlfriend went with the new beau and I went with her friend. All involved parties eventually break up. The new beau marries the best friend, the ex-girlfriend moves to the west coast to be with her online boyfriend that she's had
the whole time, I ended up with the best friend's new best friend.
TL;DR - Don't make my mistakes. The best solution is to just man up and talk to your chick. Search your feelings Walker, you know it to be true.