I am trying out the family sharing program which allows another user to acces my steam library. It works great, but he came with a problem: a friend of his lent him a physical copy of a game (Mafia II) which came with all the DLC. He says when installed the DLC is not active for some reason. I was not there when he tried it. I buy all my games digitally so I am not sure how this works - does the game come with a code which essentially means only one user can access that copy? So lending a steam-enabled physical copies of a game is not possible? If that is the case his friend (the owner of the DVD) presumably used the code to tie it to his account?

Physical copies of games that use SteamWorks are essentially just Steam codes in a box. One can't use the code and then give the box to someone else and expect for that person to be able to use it, too.

That's not how physical copies of Steam games work, sadly. One code, one owner.

@believer258: @Mike: Ok, good to know, thank you!