So, you think she'd waste money on a kinect because she's not very good at video games, but you think she'd have fun with a PS3 and Move? That really doesn't make much sense... And why would it help to get a PS3 if she's played and enjoyed Scott Pilgrim on your XBox? I know the game is available on both consoles, but I can't see any logic behind "she likes playing this game on this system, so she should get this system instead." Whether or not it's the truth, you really don't provide any good reasons why she should get a PS3, and it does kinda sound like you just want to use it, yourself. There are plenty of great co-op games for the 360, especially on XBox Live.
Now, I'm going to assume that she's done research on the kinect, move, and wii, and has her reasons for why she wants one over the other. If not, then maybe you could show her all the options and then still let her choose instead of trying to get us to convince her to go along with what you want.
Personally, I've been talking to me fiancee, and we're going to be getting a kinect soon. She's not much of a gamer, but she enjoys playing video games once in a while. Mostly, she wants a kinect for the fitness and dance programs, and she'll more than likely play some of the more accessible kinect games and have a good time.
If your big concern is her wasting money, then offer to let her hook the kinect up to your Xbox to use whenever she wants instead of buying an entire bundle. Or, failing that, help her look for decent prices on kinect/Xbox bundles.