Emma, in general, respects the opinions of Mr. Knightley and there can be no doubt that he is a superior gentleman. Yet, Mr. Knightley is free to associate with Mr. Martin as he chooses and it would follow that when/if he chose a wife she, too, would receive notice from Mr. Knightley. Emma is once again playing on Harriet's "weak mind" with a veiled threat of losing society if she chose Martin. Considering Harriet couldn't bear to be cast out of her intimate association with Hartfield she capitulates easily. The irony here is while she is manipulating her friend to appear to the advantage to Mr. Elton, she refuses to see that her friend is being badly used in turn by Mr. Elton in his scheme to be nearer to Emma...she really believes herself to be the most clever this in all of Highbury!