Hilchos Lashon Hara chapter 7, section 8

Even in a case like that of Tuviah, two conditions are required to believe the story.
a) The speaker must say that he saw the incident himself. If he only heard it, the speaker is no more to be believed than anyone else. b) Even if the speaker saw it himself, one may believe him to the extent of avoiding the person spoken of until one knows he repented. But one may not relate the story to others as we wrote earlier (4:5) and one may certainly not beat him or cause him monetary loss through this, G-d forbid.