If it's one of those games that don't require Steam to be running, you can just go to the game's directory, and launch the executable yourself. There isn't a clear description, but chances are, if there's no steam_api.dll, this can be done.
Fair enough, I think he knows that if he's automatically logged in, I can play his games. Actually, he's always been cool with that. In the past, if I wanted to play Half-Life, I asked permission to login for me so I can play it. We both have Team Fortress 2 attached to both of our accounts. Maybe I could play that.
No, you would still have to pay for the rights to play it. It would be "pre-loaded" though. I dont know much about Witcher 2, but does it allow multiple save files? Just ask him to use his account when hes not playing to play the game.
With Steam it has all the info you provide to it, including a list of games you've purchased, therefore more or less giving you a right to play it.