Does it run rough at all before quitting? When it runs, does it run ok until it shuts off? How long will it run before dying?
If it seems to be running ok then just cut out when it's being driven, I would suspect a fuel delivery issue. I would start by swapping out the fuel filter. When he removes the old filter, he could try blowing IN to the filter in the same direction the fuel flows. If he can't do that easily, the filter was plugged up.
