He’s stupid because he ordered a burger how he likes it (and probably normally orders it), starts eating it, then they ask him to sign a waiver after he’s taken a few bites?
Sorry friend, I’m not sure he’s the stupid one here. If the waiter had told him that he needs to sign a waiver before they put the order in, that’s one thing. Doing it after they cooked it to order and he started eating is where the real stupidity occurs.