Updating iPhone on other Mac

Help! My friend doesn't have Internet at his home and has a 3GS that he wants to update from ios 4 to IOS 6. He never backed up his device on his home comp. so how do I upgrade it without getting the files mixed up with my own apps videos and songs?

You don't. You can download the iOS 6 image and give it to him on a thumb drive, then he can use his home computer to backup his phone and install the iOS 6 update (just press option while clicking "Restore" and a file dialog will pop up you can point to the IPSW file.

iOS 6 IPSW [Direct Download Links]