There is definitely this idea that "the client is always right". If what you say is true, this student wanted the diploma without completing the requirements of the course. I have been teaching in physical schools in various countries and online for years and while there is a psychology to be kept with your students/clients, I do not believe that the student is always right. Udemy's policies are in place for a situation like this but I wonder whether the instructors are protected sufficiently from, as you say, someone who has the power to give you negative ratings and abuses it out of spite. Perhaps in a situation like this (e.g. a clear case that one of the parties is abusing the system for a negative reason), Udemy should contact the student and mediate with clear guidelines/boundaries. In all my years of experience, the first and basic rule of thumb to keep control of your classes and establish your level of authority is strong boundaries, fairness in all dealings, and not allowing for manipulation of any kind. I am not sure whether this helps but I wanted to participate because the world of teachers can be a very lonely or misunderstood place at times where we have all the responsibilities and not always all the benefits.
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