If you wanted serious replies this should have been put in EOEO. However I'll be serious for once.

This is just it, your mate can't keep his attention on the girl that he has so ask him if he is even interested or if he ought to just leave the girl. If he asks why you can be honest but prepare for a fight or an argument just don't lie to him, if you lie to him and then go out with his girlfriend or ex-girlfriend he's going to be completely outraged at you most likely and not without good reason.