1. If she were a true friend, she ought to have warned Anne not to trust him.
2. At the very least, she ought to have told Anne that she was acquainted with the gentleman, which would have permitted Anne to solicit additional information so as to not think herself crazy - after all, her family and Lady Russell all think he's the bee's knees (or whatever the Regency equivalent was), and Anne is alone in not quite trusting him.
3. It all feels a bit too neat, and a little like a helping of deus ex machina. At the very least it's a massive info dump.
1. Mrs. Smith indicates that she had been certain that Anne was going to marry Mr. Elliot, and there was no way she would speak ill to Anne of her likely fiancé. Since she just did it - and at length - I'm not 100% certain I buy it, but she did wait until Anne hinted broadly that there would be no engagement between them. I'm willing to accept that in Regency times, trash-talking was seldom tolerated, particularly if one is trash-talking another's intimate relations, so I'll let her slide on this one.
2. If there really is all this backstory involving Mr. Elliot's ruination of the Smiths' financial well-being and refusal to take action as the Executor of Mr. Smith's estate, then I truly don't understand why Mrs. Smith didn't at least mention that she'd met the guy before somewhere earlier in the book.
3. Remember how convenient it was that Mrs. Smith had the same nurse as Mrs. Wallis? I'm willing to buy that. But to then have this former close relationship between Mr. Elliot and Mrs. Smith, about which there'd never been the slightest intimation? I don't buy it. And I think it's a mistake, and that Jane Austen might have rectified it had she lived to shepherd this manuscript through production, particularly in light of this bit from Chapter 17, in which Anne refused to bail on Mrs. Smith in order to attend a last-minute shindig with Lady Dalrymple:
Her kind, compassionate visits to this old schoolfellow, sick and reduced, seemed to have quite delighted Mr. Elliot. He thought her a most extraordinary young woman; in her temper, manners, mind, a model of female excellence.
One assumes that Mrs. Smith's name was likely to have been mentioned in front of Mr. Elliot; even though the surname was then - as now - fairly common, his awareness that the Mrs. Smith he knew for all those years knew Anne Elliot ought to have been enough for him to put two and two together. I mean, he's crafty and wily, but not stupid.