I'm not sure contacting the buyer was the best way to go but you've done it now, just await a reply and take it from there.
Nothing good is going to come of contacting them (or so i suspect). They are unhappy with the item and we don't know why they left it for 7 weeks to leave feedback. Perhaps it developed faults after a few weeks or perhaps they are just generally slow to get around to things, including reviews. Could even be they didn't use it for a while. We just don't know.
But.. if they didn't contact you about it, then it seems they are just wearing it, unhappy but not demanding a refund. Their time to open an ebay claim has passed. That still leaves paypal but they may not know of it or think of it.
If they do reply to you, detailing a problem, you may be able to help with technical advice on how to fix it. Just keep it all polite etc as I am sure you would.
If they reply with an angry rant, saying they should be refunded etc, I would ignore it and not reply at all. Don't get into a back and forth fight.
I can imagine it is a disappointment to get a neg after all this time, especially when you feel it may be unwarranted. When we used to sell, we had the perfect 100% too but I have to tell you it was a close run thing on a couple of items with totally unreasonable buyers, whose expectations with postage costs etc were not from planet Earth. For most sellers, the occasional neg is inevitable so don't let it overly worry you. Buyers tend to look for overall trends in FB, if they look at it at all. And yours is strongly positive. I wouldn't hestitate to buy after the great comments from others. One neg won't kill you.
I think most sellers want to know about & fix any problems if they can but just make sure you look after your own interests too.
