If he can see the bootup and everything, then my first suggestion probably won't do anything (messing with the monitor options itself), but I'm not exactly sure how Windows interacts with the monitor.
I don't know if there's a config file somewhere in windows you can change, though, and I'm not sure if booting Windows in safe mode will help. It may, because safe mode is very barebones and it uses default settings for pretty much everything, so who knows. If it does display properly in safe mode, then Monk's suggestion is right on the money. Remove the Display Adapter and reboot, it should auto-detect the video card and reset everything back to defaults.