Keep in mind this responce it limited to what I've seen with laptops and the settings.
First thing that comes to mind is lap tops have batteries. They could have been dead or defective. But under normal conditions if power is lost it should have switched to battery power without any loss. Windows has an option that is set up from the factory in most lap tops that does an orderly shut down when the batteries are low. If the batteries and computer was good he should have been able to unplug the power cord and still work for a few hours.
Many laptop power supplies are have a input voltage of 100-220v. The cord that goes into the external power supply is changable to different ones for other countries and voltages. So if there was a overvoltage from a loose neutral I would think it would be ok.
As said it makes no sence for them to say they checked from another computer that a dead computer has data loss.
A volt meter will not cause any problems.
I would guess the first thing to go is the power supply if there was a power surge. Too bad they did not tell you what parts were bad.
I would not pay or admit to anything. At worst they could take you to court and or bad mouth your business.
IMO either they a tring to scam you or you just happened to be there when they had a failure and the assume it must be your fault.
