Well, I DID specify a refrigerator magnet, and those are usually barely strong enough to stick to the fridge, so I doubt they will damage the monitor.....
Oh... And unless he's got a newer-type monitor or Nintendo-style mounted in there, he's not going to have a built-in degauss function. If it's like most Galaga's, it will have a G07 or K4600/K4900 in there. But I agree it's always better to use the tool built for the purpose needed, and not just use whatever's handy....