In chap 7, Emma told Harriet that she could not even be acquainted with her if she married Robert i.e. that she would drop the acquaintance, that she would not have Robert introduced to her, she would never visit Harriet at Abbey Mill Farm and Harriet would never again be invited to Hartfield.

"It would have grieved me to lose your acquaintance ...... I could not have visited Mrs. Robert Martin, of Abbey-Mill Farm."
In her argument with Mr Knightley in chap 8, Emma had described Robert as "a man whom I could never admit as an acquaintance of my own".

In chap 55, contrary to what she had said earlier -- "Emma became acquainted with Robert Martin, who was now introduced at Hartfield".

True, the intimate friendship changed in a natural manner into a calmer sort of good-will, and I hope that through it, Emma will also get to know other members of the Martin family. She would have overcome her prejudices in chap 4 that "The yeomanry are precisely the order of people with whom I feel I can have nothing to do". While they would not be intimate friends, they would be acquaintances and there would be good-will, when previously, Emma had simply ignored people like the Martins.

There are degress of friendship and acquaintanceship. Mr Knightley had disapproved of the too intimate friendship between Emma and Harriet becasue he felt that it could cause harm to both -- as he expressed to Mrs Weston in chap 5 -- and he was right. Mr Knightley would have preferred Emma to show more attentions to Jane Fairfax, and Emma finally acknowldeged that "Birth, abilities and education have all been marking [Jane] as an associate for her". But Emma had neglected Jane (beyond the minimum civilities required), and had made too much of Harriet, until Harriet, from being humble, had grown vain in chap 48.

When Harriet's parentage became known --
"She proved to be the daughter of a tradesman, rich enough to afford her the comfortable maintenance which had ever been her's, and decent enough to have always wished for concealment. Such was the blood of gentility which Emma had formerly been so ready to vouch for! It was likely to be as untainted, perhaps, as the blood of many a gentleman: but what a connexion had she been preparing for Mr. Knightley, or for the Churchills, or even for Mr. Elton! The stain of illegitimacy, unbleached by nobility or wealth, would have been a stain indeed."

Emma's intimacy with Harriet and her plans for her had been based on a fantasy that Harriet was the daughter of a gentleman. Had Emma know of Harriet's parentage earlier, she would not even want Harriet to be introduced to her. But having known her and befriended her, Emma will not drop her, she will continue to know her, and show good will towards her and her new family. Class distinctions in society could not be broken down immediately, that would be unrealistic, but I believe the contrast between Emma in chap 7 and chap 55, shows a realistic recognition of a weakening of class distinctions.