It may not be her teeth that are the problem... it may be her palate (the roof of the mouth) that needs fixing. My daughter had a narrow palate which caused her bite to be incorrect, and she had to have a palate expander device when she was six. The device was cemented in place (to her molars) for about 4 months, and every day I inserted a "key" into a notch in the device that would tighten it very slightly. It doesn't hurt, just feels tight for a few minutes afterward. At the end of the four months, the device was removed and she wore a retainer at night just to ensure that her teeth stayed where they were supposed to be. It made a big difference in her bite, and as an added bonus she stopped snoring at night because the palate had been interfereing with her breathing. She is almost 10 now and will probably still need braces because her new front teeth came in a little crooked.
If you have any doubts, don't hesitate to get a second opinion, or to ask questions of the orthodontist and be sure you understand exactly what he/she is recommending.