Set all layouts to 640x480 max. The emulators should be at their native resolutions. Depending on what MAME he's using (I'm assuming regular MAME, not GroovyMAME), turn off hardware stretch and turn on DirectDraw video mode, with switchres on. You should turn off the resolutions that the PC monitor supports before shipping it out to him. Frankly, I don't see how you could possibly get it right using only a PC monitor.