Well, color Elizabeth (and me) unimpressed by that argument. "Jane knows, as well as I do, what Wickham really is. We both know that he has been profligate in every sense of the word. That he has neither integrity nor honour. That he is as false and deceitful, as he is insinuating." Elizabeth blushes while speaking about the Darcy family, indicating that she knows further information that she cannot or will not discuss - and you can bet that Mrs Gardiner noticed it and decided that Elizabeth is in Darcy's confidence. (She is not wrong about that assumption, although she may attribute it to a more formal relationship than actually exists between Darcy and Elizabeth, as we shall see.)
Elizabeth blames much of the fault for the elopement at her father's feet - not only did he allow Lydia to go to Brighton, but she believes that Wickham knows he won't actually challenge him (presumably to a duel) over Lydia: "Lydia has no brothers to step forward; and he might imagine, from my father's behaviour, from his indolence and the little attention he has ever seemed to give to what was going forward in his family, that he would do as little, and think as little about it, as any father could do in such a matter."
Mrs Bennet is delusional and mildly hysterical, as you might expect, full of "tears and lamentations of regret, invectives against the villainous conduct of Wickham, and complaints of her own sufferings and ill usage; blaming every body but the person to whose ill-judging indulgence the errors of her daughter must be principally owing." Kitty is in a strop over having been somewhat complicit in Lydia's ruin - she knew about the attachment forming between Lydia and Wickham and the possible elopement, but didn't say a word. And Jane has been holding everything together as best she can, since Mr Bennet is off doing what little he can to find out where Wickham and Lydia have got to.
As for Mary, she was mistress enough of herself to whisper to Elizabeth, with a countenance of grave reflection, soon after they were seated at table, "This is a most unfortunate affair; and will probably be much talked of. But we must stem the tide of malice, and pour into the wounded bosoms of each other the balm of sisterly consolation."LOL! Austen is clearly making sport of Mary and her proclamations, but Mary is, after all, just parroting the contemporary "wisdom" on the subject, essentially quoting from conduct books and the like. You are right to infer that Austen held a dim view of such conduct books, but this sort of tripe was fed wholesale to the young women of the time.
Then, perceiving in Elizabeth no inclination of replying, she added, "Unhappy as the event must be for Lydia, we may draw from it this useful lesson: that loss of virtue in a female is irretrievable -- that one false step involves her in endless ruin -- that her reputation is no less brittle than it is beautiful, -- and that she cannot be too much guarded in her behaviour towards the undeserving of the other sex."
Elizabeth lifted up her eyes in amazement, but was too much oppressed to make any reply. Mary, however, continued to console herself with such kind of moral extractions from the evil before them.
We get to see the letter that Lydia left for Mrs Forster, full of her laughter, impulsiveness, and triumph. She obviously believes she's to be married, and sees absolutely nothing wrong with running off with Wickham, having decided she's in love with him.
We learn from Jane that Wickham has left behind quite a lot of debts in Meryton, and that Colonel Forster believes Wickham to be "imprudent and extravagant" - words that imply he regularly lives beyond his means, and that may also indicate him to be a gambler.