Are you absolutely sure that was why the grade went down? (Maybe I am an idealist--but do papers really get marked down a whole letter grade due to a minor capitalization issues?)

How important is this paper? If it's a 10-page paper that counts for 1/4 of her grade, I would go to bat. If it's a minor little assignment, I would let it alone. By the way, I am a writer and editor, too.