Elinor tells Marianne all she's learned from Colonel Brandon. It has the following effects on Marianne:
1. It causes her to stop holding a torch for Willoughby, although it leaves her quite depressed. She is appalled at his treatment of young Eliza, and Marianne must now wonder whether he ever intended to marry her, or whether he only intended to seduce her the way he had done with Brandon's ward.
2. It causes her to think far better of Colonel Brandon, whom she now sees as a tragic, romantic figure. Marianne now sees Brandon as a swashbuckling hero (because of the duel), as a stand-up guy (because of his taking responsibility for the care of Eliza and for challenging Willoughby to a duel over Eliza's situation), and as a tragic figure (for his continued melancholy of nearly 20 years because of his own lost love).
Mrs Dashwood, although offstage, is represented through some correspondence and through discussion of her own grief, and of her decision that it'd be best for Marianne to stay in London (even though Marianne desperately wants to come home). Her thinking is that Marianne will be reminded too much of Willoughby if she's back at Barton Cottage. Elinor agrees with that line of reasoning. Marianne accedes to her mother's decision, with her consolation being that Elinor might get to see Edward Ferrars after all - something she thinks would be a good thing, having no idea how things truly stand there.
Mrs Jennings takes a full two days to revise her opinion that Brandon and Marianne will be married by Midsummer. (Remember, it's January.) First she moves the deadline to Michaelmas (early October), and then decides it's far more likely that Brandon will marry Elinor, since the two of them converse so nicely together. (Go ahead and place your bets now as to whether any of Mrs Jennings's prognostications prove correct!)
Poor Elinor Yes, that's a recurring statement around here. While Mrs Jennings, Sir John and Mrs Palmer are all good enough not to so much as mention Willoughby in Marianne's presence, none of them truly seem able to shut up about him when it's only Elinor in the room, so she's stuck listening to word of him all the livelong day.
Also? The Misses Steele are in town. Lucy is throwing jabs at Elinor for having come to town at all, and for staying as long as she has. The wind is taken out of her sails when she implies that they've overstayed their welcome and Mrs Jennings clearly indicates that they've done no such thing as far as she's concerned.
Miss Steele (the elder, less mannered of the pair) is all set to storm up to Marianne's room to see her, even if Marianne doesn't want company and/or is in dishabille. While it's not stated in the book, it's implied in the films (and I think it's a fair implication) that her desire to see Marianne is related to Willoughby's precipitous marriage to Miss Grey's 50,000 pounds. Lucy, already snippy because she's pissed Elinor is still in town, is only too happy to snap at her sister for her impertinence.