She should start with a beginner's book. I started out with Bastien Piano Basics series. I'd recommend this series because it teaches playing techniques and sightreading/theory. You can probably find it in a local music shop. Each Bastien book tailors to your ability. As her skill improves, she can move on to the next book. There's like a total of 6 or so books in the series, each with numerous mini-review/tests at the end of a section.
All I can say is practice, practice, and more practice. She should spend at least half an hour a day. As the pieces start to become more difficult, she'll have to spend more time each day. I usually practice 60, 90, or sometimes 2 hours each day. If I don't practice for a while, my fingers become stiff and the result isn't pretty.
And when she is ready to tackle some tougher stuff, she can go classical. Not all classical pieces are hard. Some of Bach's stuff are quite easy. Start out with slow practice and go by little sections. Trying to play the entire thing in one sitting is going to be tough, almost impossible, sometimes even for me
She may want to start immediately playing the more recent pop tunes but that needs to wait because once a person gets a feel for classical, then the pop tunes come very easily.
Also, if you don't want to put a strain on your wallet, you can get those sheet music that contain only one piece instead of an entire collection. They're fairly cheap. I have a collection Beethoven Sonatas that cost me $40 and it's only the first of 2 volumes!
Sadly to say, I've only played 3 pieces out of it, but they all range from 20-30 minutes, so the what I miss in quantity is made up for in quality.
Sorry, editted to NH for correctness. 
Should be able to play almost all pieces on that.
Linear Mode
