Mrs. Smith says this in her defense:

"My dear," was Mrs. Smith's reply, "there was nothing else to be done. 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. My heart bled for you as I talked of happiness; and yet he is sensible, he is agreeable, and with such a woman as you, it was not absolutely hopeless. He was very unkind to his first wife. They were wretched together. But she was too ignorant and giddy for respect, and he had never loved her. I was willing to hope that you must fare better."

In other words,she was being realistic. Many people refuse to believe anything bad about their loved ones and simply wind up turning on the very person who only wants to help them. Life
experience has taught her that lesson. Also,she needs help in
reclaiming her husband's property and Mr. Elliott would be more
inclined to do a favor for his fiancee than for an old friend who
ratted him out(he deserves to be ratted out).