Overkill for what she wants/needs, but if you want to build it for her...
If you're going through that much trouble, I'd go real wide range on the rear if there's any hills anywhere she might ride, to an 11-36 cass. Otherwise, I don't think there's any compatibility issues. Maybe at the BB -- you'll need to sort the spacers out, which will not be optional if the Kaffenback has 68mm BB, where the Zee is made to fit at least 73mm mtn standard.
Also, I'd pay pretty careful attention to the geometry, cross frame vs. a hybrid frame -- your wife may be looking for a more upright ride than the CX frame can realistically provide.
I know next to nothing. I am frequently wrong.