If the buyer opens a return case, you will be able to offer a partial refund, maybe to cover the cost of the repair?
Otherwise, Ebay would expect you to accept the return and refund the buyer.
Gently cleaning a ring, even a vintage item, should not result in gems falling out and splitting.
"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)