How can he knows that his girlfriend doesn't belong with him?
He'll know once he finds a deeper connection with someone else. Once he begins to feel atracted to other females.
There is no true answer as there is not enough information. For example you could say the US doesnt belong because it has no official language. Or you could say Russia doesnt belong because its not in the Americas. Or you could say Brazil doesnt belong because its majority ethnicity is not white. You need a more specific reason as to why one of these terms do not belong.