The air is certainly a very capable machine for that task. I'd say the sound, for the size, is better than expected but is only really loud enough for being right in front of the machine.
without knowing anything about this person, I'm wondering if a laptop is the way to go. does she need portability? also, most laptop screens are pretty dense, small letters. maybe a base Mini and a low DPI monitor would be a better fit? There are a wide variety of 27", 1920 by 1080 monitors. that will have some pretty big pixels, so easier to read. and a better size for watching TV, movies... Should that become a thing. Add some external speakers that it the space the computer is in. and a desktop system would be more, set it and forget it. with a laptop, you are potentially dealing with having to plug and unplug things, wireless connection issues...
alternately, going off the experience of some of my own older relatives, this might be a good fit for an iPad. built in camera for FaceTime. I've kept my parents in Macintoshes for 17 years now and they are still more comfortable with the iPad, and especially with Words with Friends. and to that, you could add an adjustable arm mount, next to the comfortable chair, so she could use it without having to hold it up.
Thanks for your input! She seems to be very comfortable with a laptop, but a little apprehensive using an iPad (her sons have them). They've talked to her about how versatile they are, but she seems to really enjoy a laptop over the iPad. Her vision isn't a problem - she's more than capable of seeing even the smallest details and has never worn glasses. I think I'm going to bite the bullet and get her the Air.
Thanks again for your input!