I agree with you both about Mrs Dashwood being remiss in the way she reacted to Marianne and Willoughby's relationship.

Mrs D's warmth and propensity to think the best of people (specifically , young men who are courting her daughters) unless she learns otherwise, is an endearing trait. It is also true that Willoughby was pulling the wool over everyone's eyes and acting like an engaged man.

However, I think the blithe way in which Mrs Dashwood made no attempt to restrain Marianne's behaviour with Willoughby amounts to a serious dereliction of her parental responsibility.

They lived in a society where a young woman's virtue and good reputation were vitally important, and the loss of either could ruin her life forever, which thankfully is no longer the case in most western cultures. The nearest modern equivalent to Mrs Dashwood's permissive attitude that I can think of is a mother who cheerfully lets her daughter go away on holiday with her boyfriend without being certain both that the girl knows how to avoid getting pregnant and risking her health, and that she is mature enough to take responsibility for doing so.

I try to excuse Mrs Dashwood on the grounds that she has led a protected and sheltered life, and is naive about the capacity of red blooded young men to act in an ungentleman-like manner, but it will not do. Even if she has never knowingly encountered any men who were less than perfect gentleman she cannot possibly have been ignorant of the possibility of a girl being 'ruined' as a result of unguarded behaviour. Even if Mrs D becomes totally deaf when female acquaintances start to gossip, she is a reading woman, and the literature of the age will have provided her with plenty of fictional examples to alert her to the danger.

Badly done, Mrs Dashwood!