I would have her do some flamingo drills. Here's a video that shows the drill. We also do it from a normal starting position from the pitching plate. I would have her start there. Also, if she straightens out her arm circle, it should balance things out a little more and hopefully keep everything moving down the power line so she doesn't tend to fall off so much. With my daughter, when she really tries to throw hard, she loses her balance and does the same thing. The funny thing is if she lets off just a little and pitches with better mechanics, it usually is harder and more accurate.
Good luck.