I don't think a monitor would do that. If it works fine at your place, next
time you re-format, have him bring over his mouse, keyboard, monitor, and
anything else he plugs into it.

If it still works at your place, I would suggest he try the PC in a
different room. Maybe his power outlet is twitchy and he is getting spikes,
or drops. Maybe the PC is near a window and it is getting too hot.