To the people who keep recommending portal: I remember reading an article somewhere that even the most obvious portal puzzles are difficult as all hell for a newbie.
Warcraft 3: This game is the first fun game I can think of with an easy and voiced tutorial - it's story is fun, it's mechanics are largely dependant on choices rather than minute-to-minute tactics and I would recommend it to anyone in a hearbeat =) But for newbs, stick to the campaign.
Tales of Vesperia: Played from the beginning, it's an interesting, fun game with a slow power growth so she can learn fairly quickly how to play properly. In addition, you can jump in as a secondary character if things get too rough for him without actually taking the controller away.
Mass Effect: The combat is somewhat easy so he doesn't get overwhelmed, and the conversation system may blow her mind.
Super Mario 64: While not a terribly short/easy game to beat (as i remember it, anyway), it's not too difficult to actually PLAY and it helps him learn the ins and outs of gaming pretty easy.
Call of Duty 4: The campaign is a blast and, like it or not, CoD is very good at giving new players a leg up. Heck, I suck at shooters (died at every occasion, from what i remember) and i still felt like a total badass.
You might notice a pattern here in that I don't recommend multiplayer mode. That's because multiplayer is HARD AS ALL CRAP whatever game you're playing - a newb is already going to be wrestling with getting her commands inputted, he's not going to want to also have to outsmart opponents who, more often than not, are going to have a better handle on multiplayer than she does.