If you check the time frame - see link below - your buyer had 30 days from the actual delivery date to open a return. That is, provided you have tracking proving delivery and, I would assume, you uploaded the tracking before the case was opened.
Ebay may allow the buyer to open a return, but they should not be able to escalate and ask Ebay to step in.
I would send the buyer a message and point out that they are too late to start a return and you will not be refunding them.
https://www.ebay.co.uk/help/policies/ebay-money-back-guarantee-policy/ebay-money-back-guarantee?id=4...
"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)