She might like Echoes of Time for the DS. (Complete title: Final Fantasy Crystal Chronicles: Echoes of Time. Eugh.)
I've played through most of the game with my girlfriend, and it's a solid Secret of Mana style game, not too hard, but not incredibly easy like the first one for DS. Basically, it's an action RPG where you both create a character, pick a class, choose a hair color, and then spend your time leveling up, solving puzzles and fighting bosses. What makes it special is that there's also a bit of a story there, but they do a good job of keeping it from getting too overblown or time-consuming. Being able to play through the story together was a big selling point for my girlfriend.
From what I've heard, Ghostbusters doesn't have split screen. Which is a shame. Maybe I heard wrong?
