What should someone who is asked about a girl for marriage, and he knows of her poor morals, do—should he tell the suitor the truth or remain silent?

If a person is consulted about a marriage proposal to a woman, he is obligated to disclose her flaws that would be detrimental to the marriage. This is because it falls under the category of obligatory advice, and backbiting is permissible in this context; in fact, it becomes obligatory for the purpose of warning. One must, however, limit disclosure to the extent necessary to achieve the objective of warning. If it is possible to warn by saying, "She is not suitable for you," or "Leave her," without mentioning her flaws, then it is not permissible to mention them.