(1) I have had the rMB for a month now, and use it mostly for writing, often hours a day. After a couple of days, adjusting typing style marginally for the keyboard became second nature. I just don't notice the keyboard now (and can move seamlessly between it and other mac keyboards without thinking). Of course, it goes without saying, that keyboards are a very personal thing. But if your GF is already moderately "light fingered" she really, really should have no problems. (Those who complain about sore fingers must have a heavier typing style than most of us have these days, now we no longer learn on sit-up-and-beg mechanical typewriters!!)
(2) If you are working with on-screen text all the time, then having the retina screen makes a HUGE difference (going back to the Air, text really does look a bit fuzzy). That plus can't be understated compared with the Air, though of course the Retina 13" Pro and the rMB are pretty much on a par for clarity. Is it worth having extra real estate on the 13"? I'd say not for ordinary writing.
(3) If your GF enjoys writing in cafés, libraries, etc. the portability of the rMB even compared with an Air is again really, really worth having. I'm surprised just how much more convenient the rMB is to take around (more so than the raw facts about dimensions and weight led me to expect).
(4) You don't say what software your GF uses for writing. On the off-chance she does scientific writing using LaTeX, I say that in that case it would be worth going for a faster chip. Otherwise, even the entry-level rMB should be a dream.