All depends on how heavily you want to use keying functions. Writing a book? Might want an external keyboard. Using it for email and casual posting? Probably not worth your while. Some people really find the onscreen keyboard difficult to use, or hate the fact that it takes up so much of the screen. For others it isn't an issue. It's an extra expense and some people find it a pain to have to carry around the extra piece of hardware. Personally I would do without but mention to her that you'll be glad to get one for her if she wants one.
I have an Ionic Bluetooth Keyboard Tablet Stand Leather Case (http://www.amazon.com/gp/product/B009L7EEZA/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1) that is very comfortable to use, but I don't use it that often -- only when inputting a lot of text. This one comes with a great case too, and the prices can't be beat.