Seems like a good diagnosis, they probably have the setup and equipment to do a better PSU test vs anything you had on hand. Testing for load is a bit tricky, you'd normally see instability during runtime once you're in Windows or whatever, it's not that obvious if you can't even get into Windows.
The current Windows install is pretty beat up so you should prepare for a re-install - but once the hardware is stable you'll be able to properly test if Windows is able to fix itself and boot up correctly, could be by some miracle it all comes back up in the end.
