Sefer Chofetz Chaim hilchos rechilus chapter 9 halacha 5

Until now we discussed speaking out to avoid a potentially bad situation from happening. Meaning, if a person was not hired yet, or there was not yet an engagement, certainly now wedding. But what if the suspicious guy started working, became partners, or even a contract was just signed as an official agreement to start or the like then can you still speak out or is it too late? Before there is any agreement or they started it’s not considered causing a loss to potential worker, nothing happened yet but now that there is a contract signed or he started working, whichever the case might be it’s very hard to just break the contract or fire him because that would look very bad for him and change a lot in his life. And the 5th condition to be able to speak is that you can’t do worse to the person than what he deserves in court. Therefore, if you know you can tell the guy’s boss and he will not trust you immediately or is not rash to make decisions and would fire the guy on the spot, but rather he would just keep a close eye on him to ensure nothing goes wrong then you can tell him and it’s even the correct thing to do. But if he will immediately take action then you can’t say a word because what you say would not hold up in court since you are only one witness. If you are two witnesses who actually saw him steal, for example, and all 5 conditions are met, then you can tell the employer and he can fire the guy if he likes because that is what would have happened in court as well. But if you both heard it second hand or even one saw him steal and the other knew it second hand then the testimony would not hold up in court so you can’t tell the employer if he will take action and not just keep a close eye on his employee.