What does she currently have in the back? Is she more or less happy with it? Does she ever spin out? Are hills difficult at times?

Assuming she's pretty happy with the current gear inches, I'd start with that and make sure she's got options for going both higher and lower. On the other hand if she rarely spins out as it is, she's not going to need much more on the higher end than that what's already there so maybe you concentrate on getting her more options on the lower side. Those are the things I'd consider.