Mine has a failing ribbon cable going from the CPU board to the ROM board... if the cabinet gets played too rough, or rocked too hard, or looked are in a weird way... or if it just feels like it, it'll reboot. I really need to replace it, but I just haven't gotten around to it yet.
Check your friends, I figured out it was the cable by the fact that when the game is up, if I BARELY touch the cable, it'll reboot. No error codes when it comes back up, just a "clean" reboot. Turn it on, let it warm up and try tapping that cable... see if that's the issue with your friends machine.
That was just my machine though, I'd probably point the blame at failing connectors (fuse holders are included in that) and possibly dirty/goof voltages from the PS. Yes, I know its a switcher... I put a switcher in a Defender last week or so, and it had been giving us problems every since (the original PS was shot as well)... put in a brand new switcher, right out of the box... fired up and has been working perfectly ever since. Never rule out the PS just because it's a switcher... they fail too.