Well Joke or not I got my wife into shooters first with Left 4 Dead with my friends, and Portals. Heck now she playing Borderlands 2. Of coarse I discovered she prefers the PC for the keyboard and mouse.
TC, the first Borderlands would be my suggestion. Easy game, especially if you make sure she plays as Lilith, and quite fun. I'm in the opposite situation, I'm a woman with a nongamer husband, but the Borderlands games are the ones he will almost always stop to watch and often comment on, saying how fun it looks and such.