It is a policy violation to leave negative comments for buyers, hence the missing ' red dot' facility. Positive feedbacks cannot be changed.
Contact eBay Customer Services but you may receive a poilicy violation stike, as you cannot call a buyer a non payer. Stating they have 100% feedback , that's because as above , negatives cannot be left for buyers, and it just increases their 100% score. Unpaid item cases is the correct procedure giving buyers that deserved unpaid item case strike.
Hopefully, this buyer has paid so explain to CS the error and you wish to leave them positive comments, hopefully that will not land you with that policy strike.
To Contact Customer Services, I find the quickest way to contact them, either for a Live Chat, or you can select to speak and request a Call Back, to click on this link:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
On weekdays lines open between 8 a.m. and 10 p.m.
On weekends they open between 9 a.m. and 6 p.m.