The security key

My father runs a website on which he has uploaded a book and the guide to open that book. To get the book you have to pay and tell my dad your machine id. In reply of which he will tell you a security code to open that book. He has also uploaded a file in which he has given the example machine id in case you do not understand what a machine id is.
One day he received a message saying 'I have paid the money, kindly let me know the security code to open it'
My father asked for the user's machine id. In reply he sent his machine id. My father replied him with the security code, but the user was unable to open the book and kept on messaging my father that its not working. Upon that my father called him and told him the procedure but still he was unable to open the book. After a toil of almost a quarter day, my father opened the guide to see what was the problem with it, and he came to know that the user has sent the machine id given as example in the guide book and no this machine id.