Snowboards my specialty.
Ok, two important things about Burton, it is you size and weight that in measured. For Burton you buy the board that is in your wieght/hight catigory.
I have Forum, and I love it. Flexibilty is beautiful, not heavy, control and preformance is amazing, and speed is off the hook. But I'm very experianced. For a beginner, I would stay away from Forum, just because they will hurt that bored, and its just not worth it. But I don't know her riding capabilities.
Generally if she is 5'5, like a 147-153 will suit her, I'm 5'7 going on to 5'8, and my Board is a 156.
A board also goes by what kind of riding you do, if she's into speed- a thinner body/width of the board. If she's into mountain- a thicker/wider body. If she's into park- a tab smaller for better spin, and overal control.
This is just a small summerization, just get all your information about her, and see what board fits best with her, get something with good graphics, and something that suits her capablities.