Let's say you're at a gathering of some sort and there's this girl there you start talking to.
Things are going well between you, then at some point she gets up to go use the bathroom. While she's gone, a guy from the group leans in and tells you, "Hey man, no offense, but I'm working on that girl."
What's the correct thing to do here?
Should you back off and stop picking up on this girl... go look for another?
Should you ignore this guy totally and continue as if nothing'd been said?
Maybe challenge him to a duel?
How do you deal with these 'working on that girl you're picking up' guys?