Have your friend replace all of the old litter with new fresh stuff. Wash out the litter tub/box. If your friend uses scented litter, try unscented.

Cats bury by scent.
An older cat might have a nose issue.

^^Thank you! Your answers combined might have just solved this mystery. We did use unscented litter once but unintentionally since my friend prefers scented litter and bought one my mistake.

So we quickly replaced it without noticing what happened, whether his cat covered his stuff or not with the unscented litter.

It’s worth trying, put down unscented litter just to see if that’ll fix the problem. Also to consult a vet about the cat’s nose. And probably my friend’s as well? Lol