I would suggest going to the music store and buying a few things:
1) A chord book. I like
The Ultimate Keyboard Chord Chart published by Hal-Leonard.
2) A theory workbook. Choose the very beginning level.
3) An actual piano course book. If you know a little about music and can help her through a few first steps, she should be able to teach herself if she starts at the beginning level. I learned off of the John Thompson books, which are targeted towards children, but very straightforward, especially if you start at the very beginning at Teaching Little Fingers to Play. The books go up to very advanced pieces if you follow them all the way through.
4) If your keyboard doesn't have a metronome on it, you might want to get one. They're helpful.
5) If your keyboard doesn't have a music stand, you need one of those, too. A damper pedal would also be a good investment, though you won't need that until later.
My tips:
1) Practice, practice, practice, practice, practice.
2) The first few months, and maybe even years, will be hard because you'll work hard for something that sounds very simple. However, this is the foundation of all the pretty stuff you'll play later.
3) Even though it seems easier to just follow finger numbers, learn to read music and to count out notes. It's really worth it.
4) If you get discouraged, get a tab from this site, look up the chords, and play them in the best rhythm you can figure out (maybe with a friend who plays the guitar). That will give you a little feeling of accomplishment.
5) Choose music that you love to play.
6) Since you have a keyboard, if you get bored with a piece, try changing the sounds and playing it.
Good luck, and don't give up! If you ever need any help, let me know