The 299th prohibition is that we are forbidden from giving misleading advice. When someone asks you for advice regarding something he is not expert in, this mitzvah prohibits misleading or deceiving him. Rather you should guide him towards the choice you think is proper.
The source of this prohibition is G‑d's statement1 (exalted be He), "Do not place a stumbling block before the blind."
In the words of the Sifri: "Before someone who is 'blind' regarding a particular thing, and is asking your advice — do not give him advice which will harm him."
Our Sages said that this prohibition includes one who assists or causes another to commit a transgression, because the person's vision is obstructed by his desire [to sin] and he has become "blind", and he comes to assist him in his wrongdoing or provide him with the ability to perform the transgression.
Similarly, our Sages said that both the lender and the borrower in an interest-bearing loan also transgress the prohibition of placing a stumbling block before the blind. This is because each one assists the other and makes it possible for him to complete the forbidden act.2 There are many similar cases where our Sages said "One transgresses the prohibition, 'Do not place a stumbling block before the blind.' " The simple meaning of the verse, however, is as mentioned above [i.e. giving misleading advice].
