How we cite our quotes:
[Sir Walter speaks] "A widow Mrs Smith lodging in Westgate Buildings! A poor widow barely able to live, between thirty and forty; a mere Mrs Smith, an every-day Mrs Smith, of all people and all names in the world, to be the chosen friend of Miss Anne Elliot, and to be preferred by her to her own family connections among the nobility of England and Ireland! Mrs Smith! Such a name!" (17.18)
Sir Walter's making a contrast here between the common and the special: Anne is special, his logic goes, and anyone she hangs out with should be special too, lest she sprinkle her specialness like fairy dust on someone who doesn't deserve it.
She could not endure the idea of treachery or levity, or anything akin to ill usage between him and his friend. She could not endure that such a friendship as theirs should be severed unfairly. (18.19)
It seems like Anne's projecting a bit here – she feels her own relationship with Wentworth was "severed unfairly," so she feels the possibility of something similar happening again especially sharply.
She learned that (the intimacy between them continuing unimpaired by Mr Elliot's marriage) they had been as before always together, and Mr Elliot had led his friend into expenses much beyond his fortune. Mrs Smith did not want to take blame to herself, and was most tender of throwing any on her husband; but Anne could collect that their income had never been equal to their style of living, and that from the first there had been a great deal of general and joint extravagance. From his wife's account of him she could discern Mr Smith to have been a man of warm feelings, easy temper, careless habits, and not strong understanding, much more amiable than his friend, and very unlike him, led by him, and probably despised by him. Mr Elliot, raised by his marriage to great affluence, and disposed to every gratification of pleasure and vanity which could be commanded without involving himself, (for with all his self-indulgence he had become a prudent man), and beginning to be rich, just as his friend ought to have found himself to be poor, seemed to have had no concern at all for that friend's probable finances, but, on the contrary, had been prompting and encouraging expenses which could end only in ruin; and the Smiths accordingly had been ruined. (21.93)
And here's how we know once and for all that Mr. Elliot's not someone you want to keep around: he screwed over his supposed friend Mr. Smith. This puts him in sharp contrast to Captain Wentworth and all the support he's given to his friends Captains Harville and Benwick.