Hilchos Lashon Hara chapter 4, section 4
However, if it is a severe sin, as committing adultery or eating things that are forbidden, and that these sins are common knowledge among all Jews, then disclosure of his actions is conditional.
If he is a typical Jew in all other respects who is generally observant and avoids most sins, and here we saw him violate a mitzvah privately only once, we have to assume that he did Teshuvah and it is forbidden to disclose his sin. However, one must privately, discretely speak to this person about the sin he committed before G-d, in order that he not repeat is action and come to sin again.
If this person is a Torah scholar – it is forbidden to continue to even suspect that he committed a sin, because he most likely did already Teshuvah. As Chazal have taught “if you saw a Talmid Chacham who committed a sin at night, one may not have any suspicions or doubts about him by following day because he most certainly did do Teshuvah.”