Is he a close friend of yours? If so, talk to him about it.
Be confident though and give your girl space. If it is anything I have learned from this experience, it is that the more you try to be "protective" over your girl, the more she will go behind your back and do things you would not allow her to normally do. Give her some space, and allow her to socialize with him. However, if you are noticing that she is starting to place a higher value with him, talk to her about it.
Relationships are based on COMMUNICATION and TRUST. Without those two, your relationship is bound to fail. Are you in a LTR (Long-term relationship) with her? If so, you should be able to communicate with her and be able to talk to her about these things. Do not confront her, or start an argument. Rather, bring it up casually at first, and simply have a conversation about it while keeping your cool.
Don't assume the worst, talk to her about it. I'm interested in what other people have to say in regards to this matter..
If you have any questions about my post, feel free to PM me.
Axcell.