Does what happened to the student—her classmate receiving the same grade despite the classmate's errors and forgetting an entire question—count as an injustice to her, warranting her not completing her studies?

Mixing in studies is a great danger. What you mentioned about the professor's behavior may or may not be an injustice to you, as a professor has many criteria for evaluating students. We advise you to continue your studies in a non-mixed class if possible, or to stop studying if you do not need the certificate. This is a precautionary measure for the sake of religion, for religion is a person's most valuable asset.