Your girl loses a bet ... so once a day for a week (or however long) she has to allow my guy to jerk off and come on any part of her body. Whether he wants to give her a facial, on her ass, on her tits, on her tongue, etc etc ...
Up to you if she wants to grow in to jerking him off, blowing him or how it grows! He'll at least want dirty talk/stories to go along with it!