Small claims court!! j/k.
Does he have a car? i assume he does, well walk up to him for and apologize and act like you and your girlfriend forgot about the whole thing. Then about a month later walk up to his ride and 1. sugar down the gas tank, and 2. Key his car. He might have taken $5 from your girl, but he disrespected her and made her cry, now your going to make him pay at least $5000.