My friend failed an exam because professor offered to drop exam and then takes back the drop and instead makes a tradeoff that the final exam is not cumulative. What should he do?

Here is a little backstory:

So my friend's professor is being unreasonable. About a month ago, the professor offers to the class a change where he is willing to drop an exam. Verbally, this policy was set in motion. Like most students would do, my friend decided he didn't have to study because the likelihood that it was getting dropped was high. He had the great idea of guessing "A" for each question as it was a bucket list of his and he ends up with a 28%. The overall average was a 36% because of the majority opting to not study or not take the exam entirely. A few days after the exam, the professor decides to take back the drop and the "tradeoff" is that the final exam is no longer cumulative and instead, on the last quarter of the class material. Since it was a verbal agreement and not stated anywhere in the syllabus, the dropped exam policy was never in effect and the professor believes that it was his right to no longer drop the exam. This screwed over my friend, who would have got an A-/B+ to potentially failing the class (not to mention he intends to go to dental school). He is currently trying to fight this, but doesn't exactly know how.

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