Possibly because some combination of his device/platform/Facebook didn't honor his request after a while. At one time it wasn't unheard of for an app to self-update and reset notifications and other settings.

or just uninstall the app without deleting your account

Revenge, probably. And/or to punish FB microscopically, but in the only way we can.

I've long used Facebook only for messaging and managing events because it's an effective and ubiquitous platform for both of these things.

Good for you. Not everyone lives the same life that you do.