Mary's remarks about Tom's illness are made in a letter to Fanny, who is at that time living in Portsmouth. Fanny does not feel able to reveal them to Edmund until after the scandal and the big break, where she uses them as a form of comfort, to help him get over any regret he might have.
Edmund makes it clear that he considers marriage to Mary impossible after the scandal breaks out, because of the family association. He could never marry the sister of the man who seduced his own sister into public ruin and disgrace. So he goes to Mary, full of grief and sorrow, expecting a final, tender good-bye, thinking she will be as horrified and saddened as he is, only to find she has a very different attitude indeed. Her callous attitude does finally open his eyes, but he still goes on to Fanny at first about how full of natural goodness she is, just ruined! ruined! by corrupt town morals. Fanny is eager to help him see Mary still more clearly, and so tells him something of what Mary wrote about. He doesn't want to believe it at first, but soon accepts it, and while it doesn't immediately cure him of his broken heart, it does help in the long run.
