This is so strange, it has to be something simple?

I don't know that much about household wattage
Neither do I, but I don't know where you live either so this is a pure guess.

You live in a domestic dwelling...........house/apartment? The repair shop is in an industrial/commercial area?

In some parts of the World there are two power supplies: 110/115 volt and 220/230 volt AC (OK you might also have 415v triple phase, but not in your home!)

At the back of the PSU there is a switch, usually with a white on red window display for 110/220 volts. What is the voltage of your domestic supply? and what is the PSU set to.

It is normally set to the highest voltage (220/230), so if your supply is 110/115 volts that may well be enough to fire up the LED, but not much else.

Please take a look at the back of your mother's PC and see what that is set to.

I honestly do not see how it can be your power supply in your home, you would have trouble with TVs, washing machines, microwaves, refrigerators and the like, if it were.