Good suggestion. If he sees the same rainbows in safe mode then it is probably the video card or a monitor problem. Doesn't sound like malware. If the problem remains in safe mode, my first extinct would be to replace the monitor if he has one available.
If it were mine I would also try disconnecting and reconnecting the monitor connections both ends. Then (if its a tower) try reseating the PC boards inside and see what happens. Its worth a shot at least. Also can you hook up another monitor to it and check it again?