I started my wife on Crash Bandicoot, this was 12 years back on the PSX.
Now, she plays everything I play, we have all the major consoles and some very rare minor ones.
Pwns me in Starcraft, Warcraft,Plays sniper support in CS, Used to be ranked No 1 on iGame Hardcore for CoD 4 and is generally as good if not better at most games we play.
My point with all of this, be careful what you wish for, there might be a natural gamer lurking under the hotness

To answer your question though, on Xbox, Try the Lego series titles if she is just starting out, co-op gameplay and a lot of fun for casual players.
We are currently finishing Lego Batman in between Resident Evil Darkside Chronicles on Wii and Left4Dead 2 Sessions on PC
And yes, as others have stated, GH is a must