The nominal utility power as supplied is supposed to be 240V. 5 volts over is a touch hot, but not out of the bounds of normal tolerances - turn everything on in the neighborhood some hot August afternoon with the AC units cranked, and tell me what the voltage reads then...
If they were seeing 250V - 255V or more, then I'd call the Utility and get the transformer taps knocked down a notch.
If the power supply on the CNC computer had changeable taps, and the last guy that touched it didn't have any reason to look to see what it was set for, IMHO it's nobody's fault. Especially if the shop they moved from and the one they moved to had the same nominal operating voltage, and they knew it - I'm not going to open 50 machines looking for the unexpected when I'm charging by the hour unless I have a good reason to... Just "Git Er Done" and go home.
If he had a reason to look inside and saw it was on the 220V tap he should have moved it to the 240V - or told the owner - it's good practice to follow but there's no responsibility to look involved.
And I wouldn't expect 255V on the 220V tap to kill it. Now if it was set for 208V input and you fed it off the 'High Leg' from an Open Delta service that's hovering around 280V to ground, THEN I'd expect fireworks. Open Delta High Leg voltages can bounce around and go even higher, then something flashes over...
That would be the /one/ time I'd call it against the Handyman, putting the high leg on the control circuit would be a big goof. You are supposed to put the regular 240V legs on the A and C phases coming in, and the 'High Leg' Orange lead to B phase and NOT the controls.
The average power supply is supposed to feed +5V, +12V & -12V etc. to the computer board, and have Crowbar protection so that's all that gets through. If the supply blows up and lets line voltage through to fry the controller board (even if you put an over-voltage on the input) that's a badly built power supply.
Otherwise, it's entirely possible that it just reached End Of Life and decided to go out in a spectacular manner, and the move had nothing to do with it. The timer that makes things blow up three days out of warranty finally went off.
Unless you want to spend a lot of money on Electronic Forensics to analyze the power supply failure, "The world may never know..."
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PS - Have to trim off alt-r.c.m to make this go, 4 crosspost limit.