Here's the problem adding a HDD when it is on will kill the system because of power surge that will most likely kill the HDD as well.
What would you expect when your friend did that it corrupted the installation thus there was no O/S installed or boot system installed.
This means your friend hosed the system and you need to start from beginning again. Your friend is no friend at this point in time. You need to reset the BIOS to default Factory to remove any changes your so-called friend did. Why are you letting the friend mess with your system? That friend has no obligation to fix anything the friend damaged.
No your friend plugging a HDD into a "ON" system most likely damaged something on the motherboard and HDD doing this bonehead move. You never plug HDD into a "ON" system do that and you can expect trouble and damaged hardware.