"You shall not put a stumbling block before the blind"—Leviticus 19:14.
It is forbidden to mislead another. I.e., if one person approaches another and requests advice in an area in which he, the questioner, is unfamiliar, it is forbidden for the responder to deceptively offer damaging counsel.
This prohibition also includes assisting another in the commission of a sin—for the one who desires to sin is considered "blind," his rational sight blinded by his impulse and desire. The same holds true for one who facilitates another's sin. E.g., in the event that money is borrowed with interest, both the borrower and lender have transgressed this sin (in addition to the sin of lending/borrowing with interest)—for each of them has facilitated the other's sin too.
