Soundds like they are out of time to claim with eBay. Has 30 days passed the item's last estimated date for delivery? Then, a case may be opened but they will not be able to escalate it. The days and time is counted to the second, in this case there would be no need to refund your buyer.
They could turn to their card provider for help with a refund, then you would need to provide any evidence you have to show the item had an attempted delivery.