If she likes mass effect, tell her about fallout and oblivion, they are all virtually the same game in different settings.
Also, I would suggest flower, portal (the orange box), and if she's into sci-fi, then she's probably into batman as well.
Secondary suggestions:
Prince of Persia
Dead or Alive (girls really tend to like this game)
Singstar/Lips
Mirrors Edge
Ghostbusters: the movie: the game
castlevania:sotn
Monkey Island
Dragon Age
To summarise: I really don't know which she should get, but overall, it seems like ps3 has a slight advantage in teh "movie games" and "indie games" department.