How old is the PC? Would it be worth re-evaluation rather than troubleshooting? Or even time saved on trying to sort it out?

I mean if you are unsure that it is definitely the PSU....it could be a voltage issue with the mobo...the heatsink stuck to the CPU issue on top, how much time do you want to spend investigating? Time is your monetary loss as well as a pc repairer dont forget....

Depending on the clients computing needs, it may work out more cost effective with less labour intensive repairs to actually get another motherboard (with onboard graphics unless he is a gamer), CPU and PSU if diagnosis is proving troublesome, providing the exisiting RAM is compatible and hard disk, drives etc are ok.