Shemiras Halashon Halachos
Illustrations paragraphs 1, 2 1. If Levi sees that Reuven wants to make a business partnership with Shimon and Reuven does not know Shimon, and Levi well knows that due to his evil nature Shimon does not care about other people's money, Levi should warn Reuven to avoid the partnership. Here too, he should observe all the conditions of the laws of rechilus.
2. If Reuven already made the partnership due to not knowing Shimon, it depends. If Levi knows that Reuven will not believe him but only be careful to not lose money through the partnership with Shimon, Levi must tell him on condition that the conditions of the laws of rechilus are met. But if Levi sees that Reuven will believe him and cause Shimon to suffer a real loss, such as Reuven annulling the partnership or causing other damages, Levi may not tell Reuven.
Illustrations paragraph 3 3. Furthermore, Levi must be very careful not to advise Reuven to make a partnership with Shimon if he knows that harm can result, for example, if Shimon is poor or dishonest. The same applies if Levi is asked about Shimon regarding marriage or work. For the whole prohibition of rechilus is to cause harm to Shimon. But to do the opposite and harm Reuven by telling him to make a partnership etc. with Shimon in circumstances where Levi himself would avoid Shimon, counts as, Do not put a stumbling block before the blind, which, the sages say, means "that one should not give him unsuitable advice." Some people transgress this because of personal gain. This is a serious transgression – love of money causes people to transgress a Torah prohibition.