In describing Mr. Knightley as having a “high and mighty attitude” are you saying he has no right to speak his opinion of Miss Smith in looking out for his friends? If Mr. Knightley spread his less than stellar opinion of Harriet freely to Highbury’s population then I would agree he ought to be wary of what he says because neither his opinion of Harriet nor the subjects of Emma’s & Harriet’s friendship or Harriet & Mr. Martin were fit for public consumption. However I don’t see that Mr. Knightley has been indiscrete or know why he should be wary of what he says to Emma (8) and Mrs. Weston (5). That Harriet is illegitimate seems common knowledge in Highbury. I don’t think Mr. Knightley is on shaky ground by saying it:

Harriet Smith was the natural daughter of somebody. Somebody had placed her, several years back, at Mrs. Goddard's school, and somebody had lately raised her from the condition of scholar to that of parlour-boarder. This was all that was generally known of her history. She had no visible friends but what had been acquired at Highbury… (3)

Harriet and Mrs. Goddard appear to believe she is illegitimate:

Her first attempts at usefulness were in an endeavour to find out who were the parents; but Harriet could not tell. She was ready to tell every thing in her power, but on this subject questions were vain. …She had been satisfied to hear and believe just what Mrs. Goddard chose to tell her; and looked no farther. (4)

Emma accepts that Harriet is illegitimate:

As to the circumstances of her birth, though in a legal sense she may be called Nobody, it will not hold in common sense. She is not to pay for the offence of others, by being held below the level of those with whom she is brought up… (8)

Mr. Elton believes that Harriet is illegitimate:

"I wish her extremely well: and, no doubt, there are men who might not object to -- Every body has their level…" (15)