ultimately, it depends on the girlfriend's attitude about your casual acquaintanceship, how long you and the guy have known each other and whether this guy you like is really worth pursuing. Some girlfriends don't like their boyfriends to have other close female friends. Not only does this strain their relationship, she may get hostile towards you (because she may see you trying to make a move on her man).
Keep in mind, if he leaves his current girlfriend for you, what is to say he won't leave you to either get back with her or another person later on?
Unless you won't interact unless you happen to be proactive, just live your life and don't give him too much thought. If they do break up, you will get a chance (though it is generally unwise to be a rebound partner).