Buyers are told to open item not as described cases where they have received something, even if it's just an empty bag. So your buyer was right.
The issue is that if you sent it using Simple Delivery the buyer should have used 'arrived damaged' as the reason for the case and then ebay would have covered the refund.
If it was simple delivery it might be worth contacting ebay explaining that the buyer used the wrong reason, but there is no policy that helps you here.
