Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 9
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אִם אֶחָד רוֹאֶה, שֶׁחֲבֵרוֹ רוֹצֶה לְהִשְׁתַּתֵּף בְּאֵיזֶה דָּבָר עִם אֶחָד, וְהוּא מְשַׁעֵר, שֶׁבְּוַדַּאי יְסִבַּב לוֹ עַל יְדֵי זֶה עִנְיָן רַע, (א) צָרִיךְ לְהַגִּיד לוֹ כְּדֵי לְהַצִּילוֹ מִן הָעִנְיָן הָרַע הַהוּא, אַךְ צָרִיךְ לָזֶה חֲמִשָּׁה פְּרָטִים שֶׁאֲבָאֲרֵם בְּסָמוּךְ.
Chofetz Chaim, Silverstein translation
If one sees that his friend wishes to enter into partnership with someone, and he feels that he will certainly be harmed by this, he must tell him to rescue him from that harm, but the following five conditions must be met: