If the issue is Fallout and that she's having a difficult time getting into it, why don't you try getting her to play Oblivion first? I mean, a lot of it is the same, just substituting hostile post-apocalyptic terrain with hostile fantasy terrain, radiation with the vampire disease, etc, but I find Oblivion a lot easier. The compass is easier to read, for one, and it's easier to keep weapons in repair. Plus the main storyline has always taken less time for me.
Also, if gaming is an integral part of your life, it's probably good to have a girlfriend that's into the same thing. Not even for the 'gaming' aspect, just that sharing interests are important. I mean, if you went to the opera every weekend you wouldn't date a girl who listened to nothing but Justin Beiber, right?