Hilchos Lashon Hara chapter 7, section 8

Even with a sin that everyone knows is forbidden, it is allowed to believe only when two conditions are met:

1. Only if the speaker told him that he saw it himself, not that he heard it from others.

2. Even if he told him he saw it himself, it is only allowed for keeping away from that person until he knows the person has done teshuva and left his bad ways. It is not allowed for going and telling others, and certainly not for causing him to lose money or for hitting him.