I wouldn't terminate this quickly because of the behaviour, but I would deffo call the parents on what you have learned vs what they have been telling you. Stress how important open communication between caregiver and parent is, and give them another chance. I think playfellt's idea of keeping on top of her behaviours will likely work if given a little time.

Just curious - why would you suggest Montessori? Do you equate it with children with behaviour issues?