Chofetz Chaim Daily 12 Taamuz 4:3 If you see an average Jew doing something wrong, even many times, you must assume it was a mistake or that he didn’t know it was wrong. 4:4 Even if one is sure that subject KNEW it was wrong, asur to tell. Maybe he did teshuva. Instead you should rebuke him privately and nicely.
Chofetz Chaim Daily 13 Taamuz 4:5 If the person you saw doing the aveira isn’t wholesome, won’t accept your rebuke & is likely to repeat the sin, you may tell someone who can influence him to repent. If two people saw, they may tell Beis Din to rebuke him. 4:6 You may tell that influencer, even if he may leak info to others.