If a person whose actions may have involved a given transgression is known to commit that particular sin at least occasionally, then the commandment, “Judge your fellow favorably,” does not require that one seek a positive explanation for what he did. However, it is meritorious to give the person the benefit of the doubt and assume that this time he did not sin. Reproof would not be necessary.
If it is absolutely clear that the person did commit a sin, then the mitzvah to reprove does apply and one should, in a respectful and caring manner, help the person to overcome his evil inclination by offering reproof.
There are times when it would be more effective for someone else, such as a close friend or rav, to reprove the individual. Theoretically, it should be permissible for the one who witnessed the transgression to inform the friend or rav of it and ask that he handle the situation. However, we have learned that one may not act on hearsay, which would mean that one cannot possibly offer reproof unless he personally witnessed the transgression. Consequently, it would be one’s responsibility to find an effective means of reproving himself, or to ask the rav to either personally investigate the situation or tactfully discuss the matter with that individual without making accusations.
Sefer Chofetz Chaim, 23 Cheshvan, page 154