Advice Without Vice
Question: An acquaintance of mine is considering purchasing a certain property and, as he has done on other occasions, consulted me for my opinion on the advisability of the deal. It so happens that I am also interested in the same property and if I encourage him to buy I will lose out. What is the right thing to do?
Answer: The answer to your question can be found in the words of the great ethicist, Rabbi Moshe Chaim Luzzato, who offers these guidelines in his classic "Mesillat Yesharim " (The Path of the Just, Chapter 11).
"This is the responsibility of the honest man. When someone asks his advice, he must tell him to do what he himself would do in a similar situation, without any consideration other than the benefit of the asker. This advice must be offered without the most remote ulterior motive. If it so happens that he anticipates a loss to himself through such advice, he should try, if possible, to reveal this to the asker. If this is not feasible he should excuse himself from giving any advice. Under no circumstances may he propose anything whose result will not be for the benefit of the asker."
The author, whose strong stand on this matter is based on the Torah prohibition "And do not place a stumbling block before a blind man" (Vayikra 19:14 ), points out that the Talmudic Sages have applied this rule to areas of life outside of business. If someone asks for information regarding a prospect for marriage, caution must be exercised to avoid misleading him with wrong information because you have an interest in seeing the other party married.
It is because it is tempting to rationalize one's bad advice by claiming it to be good that it was necessary for the entire Jewish people to hear, upon entering Eretz Yisrael, the Heavenly warning of "Accursed be he who misleads a blind man on his path." (Devarim 27:18 )