The case of Tuvia in Pesachim 113b is where Tuvia sinned and Zigud told on him to Rav Pappa and Rav Pappa gave lashes to Zigud for testifying as a single witness against Tuvia for no reason. Even in that case even though Zigud might have been a student of Rav Pappa and was trusted for what he says, that doesn’t mean it can be accepted as fact plainly, only unless there are a combination of two factors can it be accepted.
- The speaker had to have firsthand knowledge that he saw it himself not secondhand knowledge, even if he is believed as two witnesses and the person, he heard it from was believed as two witnesses still the threshold to believe it and take action isn’t there anymore. This is because a person who is honest enough to be believed as two witnesses has within his nature honesty and integrity but that can only be assumed if he is talking about what he saw himself but once it becomes second hand then maybe what he heard was a lie and even if who he heard from is honest but once it’s second hand then this level of trust is one step removed and can’t be trusted anymore to act on.
- The other factor is that if he is on the level of being trusted as two witnesses that means you can only distance yourself from the sinner until he repents. Now a days no one has such integrity of being believed as two witnesses anyways so even to be concerned and stay away from the would-be sinner until he repents you can’t do; you have to try to help him, but you certainly can’t take action against him monetarily and physically and certainly you cannot repeat what you have heard no matter how honest the speaker is. This is no different than the Shulchan Aruch in Even HaEzer 115:7 and 178:9 which says that a husband can trust one witness if he wants who says his wife committed adultery to divorce her, but she does not lose her kesuba. Only if there are two witnesses, where we reach the threshold of believability in halacha to take action does she lose her kesuba.