Reasoning, Evidence and Appeal. Those are the three main things.
Hello! According to this scenario Leslie needs to act according to policy. In the most polite way that she can she should show the customer the information and state the consequences. If this fails to work she should bring in her manager/supervisor to the situation.
Hope this helps! If you have any other questions or would like further explanation just let me know! :)
Because it make your bones stronger and my durable.