the good qualities of Robert Martin, doesn't mind that Robert considers Mr Knightley his best friend, and I am not sure that there is any evidence that Mr Knightley would not, or had not, invited Robert to dine with him (if I missed a reference to that, please supply it). I realize that Mr K is not a social reformer, but he is not as hidebound as Emma.

For Mr Knightley, Harriet has two big issues facing her -- the status of her birth and her lack of intelligence/education. Since she is not wealthy, both are a hinderance. Emma is not doing her any favors to puff her up -- there is no evidence that Harriet's father was a gentleman (which could compensate for her illigitimate status) and her lack of intellect is probably set for life -- so why get her hopes up that she can marry far above her station? My take is that Mr K is saying that it would be worse for Harriet to start taking Emma's plans for her seriously than to remain under Mrs Goddard's watch. The obviously best thing for Harriet is to marry Robert -- an intelligent and prosperous man who loves her and doesn't care at all who is her father. Mrs Goddard seems to have some standing, as she is regarded by both Emma and Mr K. Apparently not one of those who presence horrifies Emma.