Answer:
Since the patient is already very furious, she will not agree to her mistake and will tend to take an extreme action if told otherwise. In such cases one must act cautiously and more importantly strategically. The first step should be to apologize and make her feel as if she is the one at the right. Secondly, she should be treated in such a way that she thinks that you’re doing the best you can. You can try to find an alternative slot, perhaps filling her in any canceled slot.