There distinct types of money back guarantee cases.

If he opened a case for item not received then, under Simple delivery,  ebay handles everything, including the refund and the buyer would not be asked to return anything.

If he opened a case for damaged in transit, then under Simple Delivery ebay handles everything, does the refund, and the buyer keeps the item.

If he opened a case for item not as described (covers a number of reasons), this is not covered by Simple Delivery seller protection.  The seller provides the return label (or a refund without requiring a return), and when they get the item back, even if it's empty packaging, they issue the refund.

So a case for item not as described, because the item is missing from the package, will usually require the buyer to return the packaging.  Ebay does not have special rules in these circumstances. 

The buyer has messed you up by opening a case with the wrong reason.