The only thing left is to try to reflash the firmware using your friend's Mega configured as a "programmer". I may be able to help you until you reach the point of actually reflashing the firmware; the avrdude command to execute. Unless someone else steps in to help, at that point you will be on your own.
I suggest first spending some time with Google; you may be able to locate detailed instructions...https://www.google.com/search?q=atmega8u2+uno+firmware+avrdude
Bear in mind that your board and/or processor may be toast. After reflashing the firmware your board may still be unusable.