What should I do if I am asked about a girl for marriage and I mention what I know about her concerning bad qualities that she actually possesses, which then causes problems with her family? And is refraining from mentioning those bad qualities, for fear that the same might be said about my daughter, a solution?

Informing a suitor of the defects of a prospective bride, when asked for advice, is a permissible act. However, the obligation to disclose these defects varies. Those defects that establish the option of annulment (khiyar al-faskh) must be disclosed, as well as if the husband stipulated freedom from them. As for defects that entail great reproach, some scholars permit concealing them and sufficing with general phrases such as: "She is not suitable for you."

However, informing the suitor's family that you were the one who told him about her defects is a bad action that causes enmity and hatred. It contradicts the objectives of Sharia regarding reconciling disputes, for the Prophet, peace and blessings be upon him, said: "Shall I not inform you of something better than the degree of fasting, prayer, and charity?" They said: "Yes." He said: "Reconciling disputes; for indeed, discord is the shaver." This action grieves the girl and her family to no avail, and the one who commits it must fear Allah and realize the gravity of his deed.