Consequential evidence is only reliable if it is really evident that what he said is true but not if it’s partially recognizable with connecting some obvious dots or if it’s secondhand knowledge even if it seems directly recognizable still it cannot be believed, you can only be concerned and investigate.
However even if it really is clearly evident that what he is saying is true then you can only believe it but you cannot repeat it unless the person spoken about is known to be a bad person or if the issue could be a threat to others and you have an obligation to help them like a burglar or a swindler in business and you have clear evidence that this person is up to no good so you have an obligation to help other defend themselves. However, you cannot actively cause a monetary loss or physically hurt the person spoken about even if you know he did something wrong. It would seem you can boycott him, a passive monetary loss but you need witnesses and if you know someone swindled you in business you can’t just take money away from him you have to take him to court. The Chofetz Chaim says that though you know someone took something of yours you can go and get it because there is a concept of taking things into your own hands (Choshen Mishpat 4:1) but you can’t forcefully get it away from like hitting him to get it back, (though the Shulchan Aruch there seems to say you can, this needs further investigation.)
There is a case in Bava Metzia 24a where Mar Zutra Chasida hit his student or servant to convince him to admit to stealing a silver goblet from a guest which he figured out must have been him because the guy washed his hands and dried them on someone’s shirt so there was clear indication that because he doesn’t care about other’s property he must be a thief and the guy admitted. However, the Chofetz Chaim says Mar Zutra Chasida was allowed to beat him to admit though it was only based on circumstantial evidence Not two witnesses because he was a judge, judging this case and this was a special circumstance where a lesson had to be made so he was allowed to as a court in emergency purpose hit the guy to admit guilt.