It's called a "controlled jerk" (I made that up
) unless you know what you're looking for you will not see it (unless it's very pronounced). Also when I teach I stand out front (front left side) to spot this problem.
Five bucks says she's dipping/breaking her wrists at the moment of firing in anticipation of the recoil. Pointing out what is going on and then doing a lot of ball and dummy is the only answer. Also most people become very tired after 20-30 minutes of shooting and then all sorts of issues arise. The only people that can shoot with great focus past thirty minutes are people in great shape. Keep that in mind.