I think for waht its worht that Col B should have said something... but there were a few extenuating circumstances. One was the male code of honour, which a woman like Mrs S might be presumed not to share. A woman woudl be more likely to think "even if it is telling tales, I should say something since I dont wnat my friend to end up married to a sleazy man"...

Another is that Mrs S IS an old friend of Anne's... she has or shoudl have some loyalty and affection for her friend, while Col B does not knwo the Dashwoods very well and is not really that close to them BUT he has no self interest in remaining silent. What does he get from it? Nothing except possibly seeing Marianne and W getting married. Mrs S DOES Have something to gain from keeping silent and seeing Anne marry Mr E, she hopes that he will now sort out her legal affiars or that at best Anne will now be a rich lady adn she will get some kind of hlep from her...
But it is not so much that she encourages the match a bit, but she flip flops.. from saying one minute, "Oh I think he's a nice fellow and I am hoping you're going ot marry him", and in the next moment almost saying "Oh God Im so glad you dont wnat to marry him because he's an absolute so and so and I was sure you'd be very unhappy with him..."