that Mrs. Smith heard about it through Col Brandon, after Eliza confessed the name of her baby-daddy to him. I don't know if I believe that or not, but it does seem at least plausible. There is no indication of how well Mrs. Smith and Col B are acquainted, but presumably she and Sir John have been neighbors many years and she has not always been an invalid. I doubt that Col Brandon had malicious intent to destroy Willoughby's character, but he may possibly have felt that Mrs. Smith had a right to know what her heir was up to (and possibly some feeling that if his character was exposed, vulnerable young women such as Marianne and Eliza might be more on their guard against him in the future - unless it was already too late and they were engaged, as he thought later in London)