is the heatsink applied to the cpu properly too? repaste the gpu and possibly reseat the cpu heatsink and repaste that as well. if your friend is not getting NVidia driver has crashed errors or BSODs and the cpu and gpu heatsinks are properly applied and pasted then id say your friends PSU might be on its last leg
I can tell you one thing...according to your own post you tried to flash the bios of a video card that wasn't in the system. to flash the video bios you need the card still hooked up and installed in your system. just hook up the monitor cable to the onboard video instead of the graphics card and boot into windows and it will allow it there.
im leaning towards the PSU or graphics card at this point. on paper the antec neo 620 should be enough for that gear....but if its older it might be doing the dreaded voltage droop on the 12 volt rail/rails. psus do degrade over time.
it will work on an am3+ board too. phenom 2 x6 1090ts work on newer am3+ so this chip will work too. I had one of those myself. not a terrible chip, though it had a locked multiplier and no l3 cache it was an ok quad core