Is you girlfriend a big gamer at all? If she doesn't have experience with RPGs, I'd... steer her away from FF8. That game is absolutely not newbie-friendly unless you have working knowledge of RPG stats and how they work in the game. Plus, you'll be reverse-introducing her to the series by sticking her with the most black-sheep entry mechanics-wise. How is 7 more hardcore than 8? If I were picking an introduction game from the two, I'd say 7 is a lot more newbie-friendly. It's basically just a souped-up SNES game. Even if you're playing with her with 8 you'll have to stop her every time she opens her menu and outright lecture her on how that trout works.

Of your choices, I'd go with either IX or X depending on her tolerance for older graphics and freedom. If she can find her way around an RPG and doesn't mind PS1 models and camera angles, I'd go with IX since she'll be able to handle all the locations and tons of people to talk to. If she's completely new to games, go for X because it's pretty hard to get lost in that one.