Boot the computer into safe mode. It will attempt to load the most basic drivers for the hardware, including the display driver. This will allow him to re-install the video driver which would reset the resolution.
Most PCs will boot into safe mode or display some advanced boot options by tapping F8 before the splash screen.
If you two need help figuring out how to mess with the drivers, google exactly what you need, such as 'how re-install video drivers.'
Hope that fixes it.