Well, since I often try to think off-page, I admit I have considered what Fanny might have said, had she had more indiscreet manners (let's admit it, a step closer to my own tactlessness). When she stammers through these phrases to a stop:

“No, indeed, you know your duty too well for me to— even supposing—”

I might have finished for her:

“No, indeed, you know your duty too well for me to be of use in correcting you, even supposing I found you transgressing some element of it. I would not be telling you anything you had not already known, and had already chosen to ignore.”