|If the mboard has one or more firewire headers...|
Some mboards, e.g. Asus, some Gigabyte, have identical headers on them for a firewire port and one or more pairs of USB ports. The plastic below the pins is usually different colors for the two uses, but sometimes people mistakenly connect wiring to the wrong header anyway. If the wiring for a USB port or ports is connected to the firewire header on the mboard (or visa versa) , you don't have a problem until something is plugged into the mis-wired USB port(s), but when you DO plug in something, ALL HELL BREAKS LOOSE! The firewire circuits, probably the device you plugged in, and possibly the mboard otherwise are damaged in a short time!
If that's your case, the mboard is probably fried, and the thumb drive will probably no longer work in any computer.
If you don't have the wiring for the front USB ports connected to a firewire header, then the employee plugging in the thumb drive had no direct relation to the system failing - it was just a coincidence that it happened then.
State the make and model of your brand name system,
or the make and model of the mboard.
If you have an el-cheapo power supply, they are much more likely to fail, and when they do fail, they are much more likely to damage something else, often the mboard.
State the make of the original power supply.
E.g. if it's BESTEC, it's a lot more likely it may have fried the mboard.