You still have to know how to use a computer relatively well to get the most of your Mac. While iMovie is fairly straightforward to use, you still need to have a basic knowledge of video editing to understand what you're trying to do (splice together video segments, transitions,
If she's near an Apple store, tell her to take a free lesson on how to use her computer. She'll get a lot more out of it and enjoy her purchase that much more. There are also video tutorials for everything you can think to do on your Mac.
iMovie:
http://support.apple.com/kb/HT2479
iPhoto:
http://support.apple.com/kb/HT2481