1st scenario. You come back with drinks, tell your girl to give you her hands, pull, stand her up, you sit down you sit her down wherever you want her to sit. Preferably away from the guy. Make it fun. When he gets more and more of her attention and is stubborn you cut him off "oh by the way.." and talk to the girl. Now he has to jump in. That's my first guess what to do.
2nd scenario. Same thing. Move your girl away from him and cut him out of convo by starting new topic [the more personal about the girl the better, "by the way, you look like xyz, maybe not exactly like her cause you lack xyz but still kind of like cause you got xyz"].
2nd scenario. Same thing. Move your girl away from him and cut him out of convo by starting new topic [the more personal about the girl the better, "by the way, you look like xyz, maybe not exactly like her cause you lack xyz but still kind of like cause you got xyz"].