In her own reflections, Emma believed that Mr Knightley was right, and she was wrong, about Miss Fairfax -- chap 49 --

"her sense of past injustice towards Miss Fairfax. She bitterly regretted not having sought a closer acquaintance with her, and blushed for the envious feelings which had certainly been, in some measure, the cause. Had she followed Mr. Knightley's known wishes, in paying that attention to Miss Fairfax, which was every way her due; had she tried to know her better; had she done her part towards intimacy; had she endeavoured to find a friend there instead of in Harriet Smith; she must, in all probability, have been spared from every pain which pressed on her now. Birth, abilities, and education, had been equally marking one as an associate for her, to be received with gratitude; and the other -- what was she? Supposing even that they had never become intimate friends; that she had never been admitted into Miss Fairfax's confidence on this important matter -- which was most probable -- still, in knowing her as she ought, and as she might, she must have been preserved from the abominable suspicions of an improper attachment to Mr. Dixon, which she had not only so foolishly fashioned and harboured herself, but had so unpardonably imparted;".