As someone said check make sure she examines the curriculum, and teaching style to make sure it is a true Montessori and not an American hybrid. (No sense in paying Montessori prices if it ain't the real thing
.)
As mentioned, there are now many non-Montessori schools that use the Madame Montessori's concepts. For example, Montessori incorporates all the senses in learning. So, instead of just holding up a card with a letter on it and allowing visual recognition--the concept uses a card with the letter embossed and the child also earns the feel and shape of the letter as well as hearing the name. This in turn leads to the child learning to write the letter at the same time.
It was also excellent for nuturing social skills, especially if they are an only child.
My daughter attended one from 2-1/2 to 5 years old. Best thing I could have done.