Assuming that she is a complete newbie, from my own experience I'd recommend LEGO Harry Potter games, especially if you enjoyed the books and/or movies. All LEGO games are split screen co-op, fun, approachable and highly forgiving (the challenge comes from collecting stuff and there is no "game over" screen)! Other solid & fun co-op idea is Little Big Planet 1&2 if you own the PS3 - the game is extraordinarily cute and offers a ton of laughs in coop, but some sections might require few retries.

Basically, what you're looking for, are games that are:
1. easy and forgiving (the number one off putting thing for any newbie is being penalized for mistakes - forget games where one wrong key press / decision / lack of perfect reflexes equals death; for the same reason, simple controls are a must!)
2. co-operative (so that she knows you're there to help... plus, it's always more fun to play together :)
3. fun/cute and customizable (good art-style and humor does make the game a lot more approachable; also, being able to customize the character / world makes the game much more appealing for many girls :).