Sefer Chofetz Chaim hilchos rechilus chapter 9 halacha 1

If you see your friend wants to partner with someone, or hire someone for a job, or babysitter, etc. And you feel that person will definitely cause harm to your friend, for example you know he is a thief. Even if you are the only one who knows it and no one else, you still must tell your friend in order to save him from harm. However, when telling you you must meet the 5 prerequisites that will be discussed in the next halacha. When speaking up about someone who someone else is about to hire you have to take 3 things into account. (A) are you allowed to tell your friend so that he won’t hire him and cause himself a loss, (B) is there perhaps a mitzvah, obligation to tell him, and (C) on the other hand it might be forbidden to tell him because it’s rechilus since he could be causing the would-be worker a loss. All these factors must be taken into consideration before talking to your friend in order to know whether you should, could, and if so how to talk to him. The mitzvah that one would be fulfilling or not transgressing is “לא תעמוד על דם רעך” (ויקרא 19:16) “Do not stand on the blood of your friend.” This prohibition doesn’t only apply to saving your fellow Jew’s life if you can but also protecting his money as we see in Sanhedrin73a. This isn’t just a prohibition against standing by and not testifying to get back money that you know was unlawfully taken away from your friend which you have an obligation to go to court and testify about even if you are alone, not two witnesses, because you can at least force the suspect to swear and pay but even as an individual to go over to the potential victim and warn him about the thief or any damage lurking in his midst is a mitzvah and if not done one will transgress this prohibition. See Choshen Mishpat 426:1, Sifra Vayikra 19:16, Rambam Sefer Hamitzvos mitzva 297, Shaar Mishpat Choshen Mishpat 28:2, also see Sefer Hachinuch mitzvah 236).

Even though it seems very clear that one must speak up and it would not be rechilus if he helps someone not get hurt physically or monetarily but one must remember that he cannot be too quick to speak up and be sure to meet all the 5 prerequisites before doing so in order not to transgress the prohibition of rechilus.