Originally Posted by
mark7914 or
Completely reset phone, install prior known good version and disable further updates (this will take quite a long time and is quite tricky to do)
No need to reset if he's going to flash an old ROM - he'd want to reset after flashing the old ROM anyway. And disabling ROM updates requires root. It takes almost no time and isn't tricky, it's just a matter of finding the OTA app. (Disabling app updates? That can take days, and doesn't actually work - Play updates some apps that are set to not update.)