It sounds like an Air would be fine, but she might want to also consider a Chromebook.
(disclaimer: I used to work on chromebook software)
You could (both) try out ChromeOS on a Windows laptop before traipsing down to the shop to test keyboards. Needs an 8Gb USB stick that you can erase.
I'm downloading it right now and taking it out for a spin on my computer first. Never used ChromeOS before!
I used the product for the same reason: to see what ChromeOS was like.