What is the ruling on a man disclosing private details about a woman he was going to propose to, to her friend, and this friend, in turn, disclosing that information to numerous women? And what should the affected woman do about it?

If what you mentioned is true, that the man informed his fiancée of his flaws and she, in turn, informed others, then they both erred. This action is a form of backbiting (gheebah), which is speaking about your brother in a way he dislikes.

We advise patience, counseling them, reminding them of their mistake, and frequently supplicating for ease in healing, marriage, and offspring.

We also note that the flaws that must be disclosed to the suitor are those which would establish the option of annulment (khiyar al-fasakh), such as those that prevent marital intimacy, or repellent or contagious diseases.