Unfortunately, although your intentions may be honorable, your request unfortunately is technically piracy, and we cannot assist you. If it isn't on the CD sleeve, I'd suggest having your friend conduct a more thorough search for the box.
If you gave the game to your friend and your friend gave it back to you without the CD-Key, then your friend needs to purchase a new copy of the game. You don't pay for the software. You pay for the license (CD) key which allows you to play.