Friend, I think your power supply is close to ascending to a better place. Usually that's the culpit of resets and powering off.
No, if it was his power supply it wouldn't only happen while he was playing NWN.
I used to have thsi happen to me as well. It wasn't my power supply or my video card, it was my crappy onboard Realtek audio. It was unpredictable--sometimes it might happen several times a day, or it might not happen for a week. But it went away entirely when I disabled the onboard sound chip, deleted the Realtek drivers, and installed a SoundBlaster.
Try running the game with sound disabled for a few days and see if the crashes still happen.
If NWN was the only demanding application he was running, it could have been the power supply.
If it was the power supply, it would shut off on any program, really. NWN is hardly a demanding program compared to newer programs out there today. He could easily test it with all his other games and programs. If it only crashes while running NWN, it's not the power supply.
I speak from experience here. I had this -exact- same problem about a year ago. I did everything, ran virus scans, messed around with direct x, bought a new power supply, bought a new video card, checked the RAM....and none of it changed--the power kept shutting off and only while playing NWN, no other games. It only stopped once I disabled the Realtek drivers and onboard sound chip.
And disabling the sound is the simplest and easiest solution for him to try instead of buying a new power supply which he in all likelihood doesn't need.