Professor gave zero an assignment worth 20% of grade
One of my friends got a zero on an essay that was worth 20% of his entire grade because he messed up a word in his citations. He used autocorrect, which changed one of his words in the citation. This was a general education class completely unrelated to his class, and now he has a "D" in this class, completely ruining his grade. Similarly, the reason he couldn't drop out of class was that the professor entered the grades at the last moment, and some of his grades were visible on Canvas the day after the final grades were released. Also, he is not the only student to face the same problem.
What is the best course of action on his part? He has sent an email to the professor, and the professor hasn't replied to it. What is the process of doing an appeal on an assignment with some external authority besides the professor? Is retaking the same class in the next or further semesters going to replace the grade?
Lastly, do grad schools/jobs/internships care if a student has a single D in a non-relevant class?