I totally agree with others that a strong demo reel will render the film teacher’s opinion moot. I wouldn’t worry except where the B might be enough to significantly lower the GPA.
However, that is not the major problem. It’s her own self-confidence. Even when someone gets 99 percent great reviews, they tend to obsess over the one percent bad ones. I’m not sure I agree with the advice to drop the class, however. Your D will have to learn to live with some occasional disappointment. Life is like that and I think she will be stronger in the long run.