Does my telling a woman that my friend has cursed her count as backbiting? What is backbiting? And who is “al-Hallaf al-Maheen” (the one who swears much and is contemptible)?

Slander (Namimah) is the act of conveying speech between people with the intention of causing corruption. It is not permissible to inform someone that another person has insulted them, because doing so incites enmity, unless there is a fear that refraining from informing would lead to a greater harm. If one hears an insult, they must advise the insulter with wisdom and explain the gravity of their action, and that "Insulting a Muslim is فسوق (disobedience), and fighting him is disbelief." They must also defend the honor of their Muslim brother, for "whoever defends the honor of his brother, Allah will defend his face from the Hellfire on the Day of Resurrection." An "insignificant swearer" (al-hallaf al-mahin) is someone who swears frequently, is a liar, and is vile in spirit.