Revealing mistake: When Rob is in the Cafeteria eating all the coleslaw, he yells "I'm the coleslaw king of the world!" there is a girl in the background who mouths it as he is saying it. (00:50:00)
Revealing mistake: After Josie eats the chocolate cake, she starts dancing wildly on stage and does the splits. You can tell it's a stunt double that does the splits because the blond hair is nowhere near as curly.
Answer: Over the course of the evening, Guy learns that being popular isn't the most important thing in the world and wanted to make peace with Aldys. I think he was genuinely being a nice guy and wasn't in on the prank.