Pass/fail fail

Hi all! Not sure this is the right place to ask this, so forgive me if not. I’m trying to help my kiddo who is a freshman at CMU. It seems he had a pass/fail class that he failed and no one can figure out why. The professor hasn’t been answering or gave a vague answer…I’m not sure. He’s not getting help from his advisor. Is it a thing where professors can make a mistake on something like this, get kids confused or something? It’s so strange because the class was his favorite, fairly easy and not one he would’ve likely skipped or not done the work in.