The iPad is a daunting experience for many seniors. Even ones with computer experience. Even ones with Mac OS experience.
We talk about how it just does stuff, and how intuitive it is, but it really isn't for seniors. Kids get it, but they get just about anything. Seniors don't. It takes them time.
My advice would be to think about what she likes (reading books, internet forums, emails, etc.), set it up for her ahead of time with the apps, and lead her through a little at a time. For example, on Monday, show her how to do email (set it all up ahead of time) and the Internet. On Tuesday, show her how the Kindle works. On Wednesday, show her how Skype works. You can get to stuff like the app store, buying Kindle books, etc. sometime in the future. Don't overwhelm her. And, definitely don't ship it out to her in a box and expect her to get it up and running.
Remember, there is a whole lot of knowledge you need to impart. How to two-finger scroll, how to change keyboards, etc. It seems easy to us, but it isn't with them.
Good luck! I think it is a great idea. If my grandmother were still around, I would have definitely got her one. She used to love reading.