I know if i was going to buy a bike that was advertised as having been involved in any sort of accident (whether it be light or heavy damage), I would have without doubt have to have it professionally inspected before hand to save the risk of anything like this, the boy knew full well the risk he was taking in buying the bike and therefore cant expect to return it having found out afterwards that it is damaged further than he thought. He has obviously made a rash decision in doing so and seems like he's clutching at straws to make it right, u have nothing to feel guilty about what so ever!

Let him deal with it!