Musar על בבא מציעא 116:6

This is what our sages said: "if he were a Baal Teshuva (penitent), do not say to him 'remember your former deeds...' if sickness befalls him do not say to him in the way the friends of Job said: "Remember, please, who ever perished, being innocent?" (Job 4:7). If traveling merchants ask you for grain, do not tell them 'go to such and such who sells grain', and you know that he never sold grain in his life" (Bava Metzia 58b).

The fifth category is this. If the object of the gossip is a former sinner who has repented, and someone tells about the sins that he committed before he repented, in this there is great guilt (Baba Mezi'a 58b). "For one who repents of his wrongdoing, his sins now become merit" (T.P. Peah 1:1). And this gosiper shames him with sins that, through repentance, have become his merit. Moreover, he places a stumbling block before him for the victim may think in his heart, "Just as he shamed me so shall I shame him," and enter into a quarrel with him, with the result that he perverts his repentance and returns to his former state. Moreover, others who hear of this one's shame may be restrained from repenting their evil deeds, and thus the gossiper has locked the doors of repentance. And know, that if a man sees that his companion transgressed a commandment in secret and he reveals it in public, he is guilty of a sin, for perhaps the transgressor has repented of his evil way and did not want to admit it except to an understanding Sage who would not shame him, so that he could repent of his evil deed. But one should keep away from one who has done evil until he knows that his companion has repented of his evil way. And if the sinner is a scholar and a man who fears to commit a sin, it is proper to take it for granted that he has done repentance, and that if his evil inclination overcame him once he surely must have had remorse afterwards.