Any refund claims will have to go through Apple, you can't do anything. You are the one creating the product, but Apple is the one that is marketing it and selling it on their own App Store, they are the ones that process all the profits and split it accordingly, thus it is their responsibility to deal with any refund requests. I'd let the customer know to contact Apple through either contact links in iTunes, or the "report a problem" feature in their iTunes account.