Get it independently checked. If they decide it's bent then give him his money back (as you have already said you might do) remember it could just be the fact that he's realised that it's too fast or something as a first bike or he doesnt like the riding position (after riding it) and now wants something with a more upright riding position like a bandit etc. Check the mileage on it when they bring it back as if they've done hundreds of miles then you can tell them to jump.