. Could it be possible that the power shut off and came back on while the compressor was running? The compressor won't start up under a load which is why when we unplug it, we have to let it sit for 15 minutes before plugging it back in. This lets the freon flow through so the compressor won't be trying to start up against that pressure. But if the lights were really out, then this might not be the case. I guess you wouldn't be able to check this since he already unplugged it, so you wouldn't see it starting up every few minutes until the overload tripped.
. Now does until have an electronic display on it? Were those lights off as well? If so, could it be something else, that shut the whole fridge off? Maybe a breaker was tripped. Maybe a GFCI outlet had tripped. In older houses the GFCI might not be in the same room, when they were expensive (1980s or so) they would have one for the whole house. Might be next to the breaker panel, but could also be in the garage, kitchen, or bathroom. So it blows in the bathroom, and he resets it without thinking. If the lights were really off, this is a pretty likely theory. Or maybe the outlet, or half of the outlet is on a light switch, and it got switched off for a bit. I would go on this theory until you see it again. If it does go again, make sure the customer doesn't touch anything until you get there and check the power, as he might not even be aware what is hooked up with that outlet.