What happens to a friendship when a friend of the opposite is in a relationship?

It all depends on how strong is your friendship with these friends , And what u r saying is a bit scary because you shouldn't worry you at all because no women really in right mind would ask her boyfriend to choose between you and him , and if he chooses her over you then he was not real friend . Girlfriend /boyfriend relationship last for while not as good and bond of friendship which stay forever i hope my answer could be of help for you