what she means: In the Prisoner of Azkaban Remus Lupin tells the students that no witch or wizard has ever seen a boggart's true form, but in Order of the Phoenix, Mrs. Weasley asks Moody to check if the cabinet upstairs contained a boggart reason being that he has a magical eye that can see through walls. That means that the boggart couldn't see him so it couldn't transform into his worst fear which means that Mad-Eye Moody is the only wizard that we know of in all of time that knows what a boggart actually looks like but it's never mentioned by anyone ever again.