Can I transfer games from my friends steam account to mine? [duplicate]

You should be able to do the backup of the game and then restore it on to your machine via the Steam UI.

I have successfully done it by just copying the folder that contains the game and transferring that via USB drive. Probably quicker, but it will have to verify the files when you try and 'install' it on your computer, but that is usually a quick process if his game files are up to date.