Khalsa Montessori, a charter school, is supposed to be excellent, and near a lot of very nice neighborhoods. My sister and her family are moving to phx next week, and her boys are on the waiting list for Khalsa. There are other charter Montessori schools that are supposed to be excellent. Somewhere online, don't recall where, your sister should be able to find a list of all schools and Phoenix, and their status, which is based on test scores. I know standardized testing isn't the only true measure of school excellence, but it does say something.
We live in the Prescott area and our kids go to two different charter schools. Our son is no at the same school as his sisters, due to lack of space, but will be next year. He's at a Waldorf charter, and we're satisfied with it for now but not thrilled. We are, however, quite thrilled with our dds' school, which takes a Multiple Intelligences approach. Their teachers are absolutely wonderful, as is the administration. The school maintains Excelling status without "teaching to the test." Hmm, I just went off on a tangent, but just saying that there are some wonderful schools in AZ if one just does the research, visits, observes, etc. Best wishes to your sis!