It’s probably some external change since internal changes (audio chain components) do not happen according to the 24 hour clock - worse dinner time then problems ends at the latest midnight. Therefore the obvious eternal influences are power and internet connection, or somehow EMF/RFI spikes periodically in his living zone.
Seems the only external influences come from RFI, EMI, power line, internet connection. If not within the home, then it’s the power line or the internet connection. Both peak times coincide when people use both more power and personal internet resources, dwindling as people go to bed. It can also be some business using more resources, like a restaurant, illegal manufacturing lab, Internet cafe.
If it’s the internet connection, then maybe change carriers or change carrier line if possible.
If it’s electrical, then maybe get your power quality tested. For crappy power, Stromtank or solar panels + battery storage system to at least get your system off the grid. Maybe balanced power conditioner like Equitech o EquiCore might help. Or a power generator like PS Audio. Good luck.