Rambam, Laws of Temperament 6:7

If one observes that another committed a sin or walks in a way that is not good, it is the person’s duty to bring the erring one back to the right path and point out that he/she is wronging him/herself by this evil course, as it is said, “You shall surely rebuke your neighbor” (Leviticus 19:17). One who rebukes another, whether for offenses against the one who rebukes him/herself or for sins against God, should administer the rebuke in private, speak to the offender gently and tenderly, and point out that the rebuke is offered for the wrongdoer’s own good, to secure for the other life in the World to Come. If the person accepts the rebuke, well and good. If not, the person should be rebuked a second, and a third time. And so one is bound to continue the admonitions, until the sinner assaults the admonisher and says, “I refuse to listen.” Whoever is in a position to prevent wrongdoing and does not do so is responsible for the iniquity of all the wrongdoers whom that person might have restrained. [Freeman translation]