Hi, the joys of teaching:)
Gaslighting is not the case here. You did not intentionally manipulate for your gain.

As the prof, you have the right say, “Sorry, Abbey “ and leave it there.
Optionally, offer a second chance and rewrite for a better grade.

The concern is diminishing or dismissing another’s feelings which Abbey felt when she heard, “I hardly think one C is going to destroy you.”

This is why she used the word, gaslighting. She didn’t feel heard. Her reality/fear denied.
She was stressing over not getting into Browns. At that age, it feels like the end of the world.