What you said doesn't really make sense, but I think I know what your saying.
This happens to me alot and the only thing you can do is stand by, be friendly to the guy, and act like it doesn't bother you (which it obviously does from your post). Don't be silent, make her and him feel your presence, but don't act like your upset. The worst thing you can do in a situation like that is show insecurity. The way I think about it is she isn't doing anything wrong by talking to the guy and hes not doing anything wrong by talking to her. If he makes a move on her or in any way disrespects you, then you be a man and tell him to **** off. I also like to think about all of the other girls I talk to and look at on the street and then it doesn't seem like a big deal.