Dude, you did way more than you needed to do by even checking to see the coil was bad. Anything after that he is EXTREMELY fortunate.

You don't know how he's driven it, or how many miles he's put on it since he bought it. I can't possibly see how giving him a full refund is fair. If anything, tell him you'll give him a refund minus the cost of a rental car for every day he's owned it. He may end up owing you money HAHA.