Everyone is different, remember that. I certainly can't disagree with the resounding call for KotOR, but only if she likes star wars.
Avoid anything that requires her to master camera control (KotOR almost plays itself, so it's not so much of a drama). Most new gamers (and especially women) have difficulty manipulating multiple things within the simulated 3D environs we all thrive in. Older games are better for that, being stuck with an isometric camera, or side-scrolling. If she wouldn't mind the dark setting, someone else suggested Diablo II, which is incidentally the first game my now wife got into.
If she's a real girly-girl, you're going to have a greater difficulty getting her 'into gaming', because what you will consider a good game will either have too steep a learning curve and/or not interest her in the slightest. To contribute something else, Sid Meier's Pirates! has a good combination of different game elements. The boat and sword combat are both easy to get a handle on, the game also has a little trading and romancing governors' daughters is a little rythem based minigame. That's another one my wife has really enjoyed.
Oh, and my wife is currently bashing her way through Dragon Age, which I described as "kinda like a 3D diablo"