Simple Delivery only covers the UK leg. The International leg is covered by the terms for the GSP under which you are not responsible if the item is lost or damaged during the International leg of the journey.
Your buyer needs to open a return request using the reason 'Damaged in transit'. You then need to respond to that citing that you are not responsible under the terms for the GSP and eBay should then refund the buyer themselves without the item needing to be returned.