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maybe the radiator thermo switch, try unplug it to see if fan stops
Actually the opposite. He should check if the switch is unplugged. With it unplugged the fans will run constantly with the key on. A broken wire or other fault in the switch could also cause the fans to run all the time.
I don't have my owner's manual here to double check, but I recall reading that any fault in the fan control system will cause the fans to default to running all the time on high.