I see no reason why you should issue a refund given the circumstances, other than maybe just playing the good guy. I know of cases when people asked for refunds when someone downloads a model with 2 million polys but their computer can't handle such a big model so it doesn't open it :) If you're working in 3D you should at least know the limits of your software before you buy models. That's my 2 cents.
Anyway, if you'd like to play the really really good guy you can just do the conversion you mentioned yourself and give the new files to the buyer. If after that he still wants a refund it means the problem is not the model ...
P.S. Oh an there's the chat at the bottom right corner of the page. They can help you with anything you ask them, during work hours of course.