Well... It really depends how much that girl is flirting with your guy. I mean, a simple greeting and a little laugh is all right but when she goes to the extreme to the point that you know that what she is doing is stealing your guy. But this is kind of a lose-lose situation. If she wasn't really hitting on him you might get labeled as "Over-protective". If she was actually hitting on him yet you ignored you might get labeled as "Not caring". But maybe you're just misinterpreting it. Maybe she's not really flirting. Elaborate you're situation and maybe then we'll find an answer.