I've seen something like this before. Yes, the lowest resolution I recall seeing is 640x480, but I've run across a few games that actually call for a resolution of 320x240. Windows handles this by zooming in on the upper left-hand corner of the screen when a game of this type is run in fullscreen mode. And then when the game exits, Windows zooms out again. For games that crash, this zoom out never occurs, and the user is left with the blown-up view of the desktop. Three-fourths of the desktop is simply offscreen! As such, the start menu is inaccessible by mouse, as are the OK, Cancel, & Apply buttons on the display properties.

This is what it sounds like to me, and if it is, do not forget the Tab button. Tell your friend to make the changes to the resolution & colors (those should still be on-screen) and then tab to the OK or Apply button. Yes, he will be tabbing blind, so it could be a little tricky. But it's either that or a reboot (Alt+F4 from the desktop should do the trick).

I just thought of one other thing--it MIGHT be possible to just hit Enter after making the changes to the resolution and colors, eliminating the need for the Tab button.