Hilchos Lashon Hara chapter 4, section 4

But if it is a serious sin, like forbidden relations or eating forbidden food, which every Jew surely knows is forbidden, then it depends on who the person is.

If they are an average person who usually does not sin, and only once sinned in private, we assume they probably did teshuva (repented), and it is forbidden to tell about it. One should only rebuke them privately, so they do not sin again.

If they are a Torah scholar, it is forbidden even to think that they did wrong, because they surely did teshuva. As Chazal (our Sages) said: "If you saw a Torah scholar who sinned at night, do not think badly of him by day, for he surely did teshuva."