There is nothing wrong with the card, nothing at all, cards dont just downclock themselves and only work half arsed, if theres no artifacing, theres nothing wrong with it, its like saying an overclocked 5850 is better than a 5870 - it IS, because its overclocked.
That's pretty much what I've said to him.
If he is set on returning it then accept it back. Make sure its the exact card you sent him, if you kept logs of serials etc that would help but I doubt you did as I never do! When you get it, test it and sell it again on eBay - fari enough you will lose the delivery costs but that's all.
I may have to do this, however as Paul says im worried the card i get back will be different or botched in some way. I never kept any logs, i just know what it looks like.
I'm worried that if i accept it back, it now won't work properly and ill be down £85 with a broken card.