Your friend has damaged his amp. The power light coming on means that the amp's power indicator LED is at least working. That light is only there to tell you that the amp is receiving power on it's 12 volt rails, not that it's necessarily using the power in any way other than warming up the amp's internal circuitry. When you say the amp gets power, I'm not clear on whether you mean the amp works in that fashion, or there's just a red LED glowing but nothing else happens. The amp needs to be repaired, and I would suggest to him to not apply power to it again until it is checked by a professional. The power supply has probably been damaged, and it's possible, depending on how the amp was mounted, that it was searching for ground through the amp's chassis if it was screwed into metal at its mounting location. All that seems certain as of now is that the LED works, but nothing else, unfortunately.
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