Laws of Rechilus / 9: 1-2 (106)

1. One who knows that his friend wants to go into partnership with someone, and he assumes that it will surely cause him harm, he is permitted to tell him, so as to protect him, under the following conditions:

2. These are the conditions: A. He is certain that it is truly harmful. B. He must not exaggerate. C. His intention is for a constructive purpose. He must be certain that his friend will heed his advice, and not go and tell the person]. D. There is no other way to effect a solution. G. The report will not cause the subject any harm, but only prevent the partnership.