This is not an issue that Ebay can deal with - they are not responsible for shipping or delivering items.
If the tracking shows delivered, the usual advice is to ensure that you sent the item to the correct address, as shown in order details on Ebay, and then suggest the buyer check around their property, ask neighbours etc.
It is possible the parcel was delivered and then stolen.
Has the buyer opened an 'item not received' case?
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)