Cause of death?

[Flashie]

A friend called me to pick up their PC that had died after one of our daily country-wide blackouts.

Don't go thinking immediately that this is the cause though.

Knowing about the blackouts, he removed his PC's power cable, which he plugged in again after said blackout.

The PC would start, spin the fans really fast, but output nothing.

 

I disassembled the whole PC to the barebones, and tested using known-good equipment (PSU, RAM, even CPU, over both on-board and dedicated graphics, separately and together), Cleared the BIOS, replaced CMOS Battery, and was able to pinpoint it down to a dead motherboard.

 

I informed my friend, and he has in the meantime bought, and is waiting on, a new Ryzen upgrade kit (RAM CPU and Mobo).

This was all during Tuesday evening and Wednesday morning.

 

Today (Thursday), during one of the blackouts, I reassembled the PC using the dead mobo, while waiting on the new parts. 

After the blackout, I turned the PC back on, Just to find it now miraculously working again and giving an output over onboard graphics.

I restarted the PC, and after restarting, It was dead once more.

 

What is the most likely cause of this that i should inform my friend of? His house does have earth-leakage/grounded electrical.

Thanks

 

tl;dr Mobo died, But PSU, CPU, RAM and HDD's are all fine. What's the cause?