Agreed. Although it does depend on the circumstances of the day. There may be a compelling and genuine reason to divert because of adverse weather. I think the main worry is that people will use the concession frivolously and although we don't know the exact circumstances of the day in question it certainly didn't sound like an unwarranted diversion.
Maybe the airfield used for the diversion might listen to a case as per CFI's post. Perhaps they will take the view that it was just a normal factor for the day, in which case the student should have paid.
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Get there eventually