I'm not a huge Mrs Smith fan, but look at it from her point of view. She hasn't seen Anne in 12 years:

"Twelve years were gone since they had parted, and each presented a somewhat different person from what the other had imagined." (ch 17)

That's a very long time, and Anne was just 15 yrs old when last they saw each other. How much do they really know each other now? They are just beginning to be reacquainted, we know of only four visits specifically mentioned in the book, though of course, there may have been more.

It would have been incredibily impertinent of Mrs Smith to meet Anne again after all these years and say, "Oh, by the way, your cousin Mr Elliot is a man without heart or conscience." She has to get to know her again before something like could be said. And when she does begin to know her again, Mrs Smith is led to believe that the engagement is a done deal.

"I considered your marrying him as certain, though he might not yet have made the offer, and I could no more speak the truth of him, than if he had been your husband." (ch 21)

What can she say then? "Oh, by the way, your fiancé is black at heart; hollow and black!" After so many years apart, the shortness of the reacquaintance, they are not on that kind of footing with each other that would admit to such warnings.