If he can not see the device on the pc then he has no chance to flash, this is why in the beggining i said he MAY have a usb fault and the more i read the more i suspect BUT i do not know why if he has tried a fully charged battery from his friends mobile why it does not turn on.

Unless he has totally wiped the boot sector of his device but that should not stop the device from being seen on the pc, so my thoughts without seeing it is the usb port is faulty, or his usb lead of course and also he may have another hardware fault, very strange situation.