Need advice on touchy subject

Hey guys have a problem I have a friend "John" and he has a 1983 240D with 223,000 miles on it with no working AC and a few rust spots on it but the paint looks good and such and no service history other than the salesmens promise that it had bee taken care of at their dealership. He bought this car a few years ago against my advice for a lot of money 7K or so if I am remembering correctly. Now "John" wants to sell if for 6K ish......my question is how do I tactfully bring it up that his car isn't worth anywhere near that much without hurting his feelings?