I got my friend Peggle as a Steam gift and she was addicted to it for ages (as I expected, since my Sister was aswell). Now she regulary downloads the demos for the indie games...never buys the full ones as her attention span for them is pretty bad and moves on quickly. I can tell you now though she also loves to play Gears of War in co-op...even though I think its kinda meh, I'll happily play it with her since its the only one she seems to like...its available for the PC but we play it on the 360.
Others that spring to mind from a few of my other friends is tbh what has already been mentioned...rollercoaster tycoon and the sims.
I know this is like...a PC gaming forum but you should look no further than the Nintendo Wii for your more female friendly gaming.
P.S
We let my sister loose on DOOM III before, she had a rather large minigun and when something scary run at her she panicked, got the flashlight out and started bashing this scary thing with it. Then she died (obviously) at this point and never played it again, but it was a good moment.
She's also played TF2, she admits to liking it and its easy to get into. She first went as a pyro, which if I am honest is the easiest class to use but she said she likes burning people...in which case me and her fiance smiled at her in a rather confused way. So at any rate TF2 seems to be the only online game she'll touch.