Fanny has been amused many a time against her conscience when with Mary.

and deriving no higher pleasure from her conversation than occasional amusement, and that often at the expense of her judgment, when it was raised by pleasantry on people or subjects which she wished to be respected.

And there is one other conversation where Mary assumes Fanny thinks as she does.

There is nothing frightful in such a picture, is there, Miss Price? One need not envy the new Mrs. Rushworth with such a home as that.” “Envy Mrs. Rushworth!” was all that Fanny attempted to say. “Come, come, it would be very un–handsome in us to be severe on Mrs. Rushworth, for I look forward to our owing her a great many gay, brilliant, happy hours. Chap. 22

Where Fanny thinks it would be a presumption in her to envy Mrs. Rushworth, Mary interprets it the other way- that being married to poor Mr. Rushworth would be far from an enviable situation. Of course, privately Fanny would think the same thing, but it wouldn't be proper for her to acknowledge that to Mary.