I'm currently building a PC for my mother that is basically tinyurl.com/falconguide "minumum" with a few minor changes.
Since I don't live close to her and can't make sure that she doesn't install loads of bloatware, clicks malicious links or let's my little brother install things on it I thought about making her use ubuntu.
She has barely used PCs in her life outside of editing spreadsheets at work and she has no attachments to a specific OS. She will be using this computer for bookface, youtube videos, maybe watching DvDs and text editing. Is Ubuntu a good option in this case? I find Unity to be really baby friendly.
I have met quite a few people who are far from tech savvy, 4 of them now are using ubuntu just fine after seeing it on my computers and then wanting it. They usually require about an hour of your time for you to teach them how to use it. Unity looks beautiful, they actually want to learn how to use it, then show it off to their friends.
If she is real tech-inhibited like my father is, maybe you should make shortcuts in the menu for the most likely to be used programs. Make it as easy as possible and give her a slow and guided tour of the most essential things of the operation systems (text-editor, media-player, software-center, internet browser and how to turn it on and off)