Buyers can only get a return via eBay or PayPal if they report a problem within 30 days, so your buyer can't claim from eBay, nor can they claim from PayPal, who also have a 30 day claim window.
If they funded the payment with a debit or credit card they have 120 days to issue a chargeback via their card provider, and it looks like they're past that now.
So, you can safely message them back and let them know that they've had the item since the beginning of December, and you are unable to do anything now that four months have passed as there were no problems with it when it was sold.
Add them to your blocked buyers list here:
https://www.ebay.co.uk/help/selling/resolving-buyer-issues/blocking-buyer-ebay?id=4082&st=3&pos=2&qu...