I'd never do it personally. Even if it were an ex of someone that used to be my friend. Two friends that I used to hang out with said that if their girlfriends were to ever date one of their friends, that it'd be me. It's a matter of honor and respect(for myself). If you still have doubts you shouldn't act on it at all just yet and clarify things with your friend just like DeadlySin suggested.
Good luck to you!