Hilchos Rechilus, chapter 9, section 1

Earlier, in the laws of lashon hara, chapter 10, we explained when it is allowed to speak lashon hara about someone who behaved wrongly toward another person, when the speaker's goal is to help. Now we will explain when it is allowed to tell rechilus from the start, when the speaker's goal is to prevent harm.

If someone sees that his friend wants to go into a partnership with a certain person, and he thinks this will cause his friend harm, he must tell him in order to save him from the harm. But this needs five conditions.