Is what the woman said—about another woman seeking to marry a man, then seeking another young man after the first one's engagement—considered a slanderous accusation (qadhf)? And what is the ruling on that?

A woman's statement that a certain woman is seeking to marry a specific young man is not slander against her, for slander (qadhf) is to accuse someone of fornication. A woman's seeking to marry a righteous man, within the Shariah-compliant controls, does not involve an accusation of wrongdoing; rather, it is permissible for a woman to offer herself to a righteous man. However, this statement might fall under blameworthy backbiting (gheebah), which is mentioning someone with what they dislike. Its expiation is repentance to Allah and seeking forgiveness from the person whose right was infringed, if possible, without causing further harm.