The solution is to trust her...
People don't own other people. People have free will. She is with guy A right now because she wants to be. No one is guarenteed permanence. Keeping her away from other guys, doesn't guarentee permanance. If guy B is trying to move in on said girlfriend, and girlfriend doesn't see it- well then Guy A has nothing to worry about. Girlfriend sees guy as a friend... Nothing is gonna happen. If girlfriend's head is turned and now girlfriend is into Guy B - well there was a problem with the relationship with Guy A and he should be glad he found out sooner rather than later....
It makes me absolutely crazy when people try to tell another person who you can and cannot be friends with... My parents tried to do it to me in high school, when two of my buddies didn't have the best reputations.. trust me to do the right thing... I always did.