|Some video drivers have a "virtual desktop" or similar feature. The virtual screen size (resolution - width x height) is larger than the physical screen size (resolution).|
E.g. a 1280 x1024 virtual display on a 1024 x 768 or 800 x 600 display.
The display stays stationary when the mouse cursor is not near the edges of the display, but when you move the mouse cursor to the edge of the screen, the whole display moves in the same direction you moved the mouse to.
That feature was common years ago for some PC video adapters, e.g. ATI video drivers had that feature, they called it a virtual desktop, but not many video drivers these days have that feature.
(A friend of mine discovered by accident that XP has the built in ability to do that, at least regarding the video drivers that were loaded on her fairly recent computer, but I didn't take note of how she did that.)
If that sounds like your/his situation, examine the settings for your video drivers to see if something similar is enabled, and/or look at the Help info for your video drivers in the program itself, or search for info about that for your model on the Apple site
Otherwise, I can't help much with that - I know nothing about Macs.
If that doesn't help....
You could try starting a Topic in the mac forum on this site, which has little activity, or post about that on a busier mac help site elsewhere.