First of all, thanks to everyone who replied. There are a lot of good ideas here. She's actually excited to see so many people taking the time to help.
Second, to everyone saying that I'm overthinking it, that games aren't actually that hard to learn, and that I should just drop her into an easy game and let her sink or swim... Well, the issue is less that I don't think she can learn, it's more that she is intimidated. She's convinced that she has no coordination and will need weeks of intense training to take on a game as complicated as the ones she wants to play. And if she doesn't improve quickly, I don't want her to get discouraged. Hence the parameter that I want games that are easy, simple, and low-pressure.
Third, it looks like Portal is my best option. I should have thought of it, but then I've never played Portal, so thanks for reminding me of it. I think it will offer the best mix of low difficulty and fun atmosphere, and will be a good way to get accustomed to a 3-D environment.