Hilchos Lashon Hara chapter 4, section 3

This law of judging a person favorably can be divided into specific details. If this person is average, just like a typical Jew, meaning that although he is generally observant he does occasionally slip and sin and one can assume his slip was unintentional or that he thought that it was only a stringency that one could electively take upon himself if he decided. In this case even someone who saw him transgress this law several times is not allowed to disclose his actions, and it his forbidden to hate this person and he is obligated to give him the benefit of the doubt according to some Authorities.