I had a guy buy my go pro. Same story as you, all working until a week after he received it. Sounded suss as he had waited a full week to tell me ot was not working. He asked for a refund. I refused and suggested the first of all he try a firmware upgrade, then if that didn’t work it could have been damage in the post and in which case he could claim against the carrier as it was insured.
The winning move for me was when he agreed it was probably damaged on transit. So I offered him the details by which to claim against the post office. Nothing from him for a week then an email saying the firmware upgrade worked, it was all fine and sorry for bothering me!
Lesson? Don’t just rollover and offer refunds at the first sniff of a problem.