You are covered if you have tracking to show delivery to the buyer's address.
Unfortunately, and we hear this all the time here, items are stolen when couriers / posties leave items in unsafe places.
Was this a Royal Mail delivery, ask your buyer to ask the postie where they left the item ,and to perhaps suggest a safe place for future, as this seems to be a common event.
End of the day, you do not have to refund the buyer if tracking shows delivery. They could try their home insurance? Some peeps have found they have such insurance cover.
@metalfox1986