Is warning one's wife, relatives, or friends about the bad behavior of certain individuals, and mentioning situations and actions they have committed, considered a sin, backbiting (ghibah), tale-bearing (namimah), or slander (buhtan)?

Warning others about sins does not necessitate mentioning those who commit them. If they are mentioned in this situation without a [valid] benefit, it constitutes forbidden backbiting. However, if advice cannot be given except by mentioning the person's name, then it is permissible to mention them only to the extent necessary to convey the advice, without slandering their honor.