Kind of a tricky situation. Honestly I don't think it's fair for someone to forbid a friend from going out with someone who they don't go out with anymore. Then again, I would hate seeing one of my friends with the current person I'm with, if we ever broke up, but the fact is, I would have no control over what that person done anymore because we wouldn't be dating. If my friend really wanted to date them, who am I to say no? It may be hard to see, but it's a sacrifice one should make for a good friend I think.