It's the buyer who has used the wrong reason for the return.   If they contact ebay they will change it for them.   

They need to use "Damaged in transit".   Then ebay will issue the refund themselves, you keep the payment and the buyer does not have to return the empty package.

For the buyer to contact ebay send them this link:

https://www.ebay.co.uk/help/eua?id=5190

That has the options to ring ebay or have them ring the buyer.    But it must be viewed on their phone.    The options do not appear on a laptop or PC.