When you get to hook up with the friend of the girl that your mate is going for, because you and the girl friend are stuck with each other and there's not much else to do.
It's not a great reward but better than going home empty handed.
Note that it's not a wingmans' reward if you're actually into the girl. That's tag teaming and not wingmanning.
I wingmanned Kyle last night and collected a half decent wingman's reward.