I come up with the same problem quite often and I don't know what to do either .
SOS Help ?
Refunds are handled by Apple Care. There are no StoreKit API's to support refunds through the app. Please refer these customers to contact Apple Care.
rich kubota - firstname.lastname@example.org
developer technical support CoreOS/Hardware/MFI
>we need to refund their in-app purchase.
Don't do it. Refer them as Rich notes and cease communication. Such requests are frequently nothing more than fishing expeditions, looking to defraud the dev.