Сифрей Мицвот к Дварим 13:19

It is mandatory that the hand of the seducee shall slay him, even as it is said: "Thy hand shall be first upon him to put him to death" (Deut. 13.10). The seducee shall not be-friend the seducer, as it is said: "Thou shalt not consent unto him". Moreover, think not because it is said of thine enemy, "Thou shalt surely release it with him" (Ex. 23.5.) thou shalt also aid this one, remember to learn another saying: "Nor hearken unto him" (Deut. 13.10.); think not because it is said: "Neither shalt thou stand idly by the blood of thy neighbor" (Lev. 19.16.), thou shalt also not stand idly by the blood of this one, remember to learn another saying: "Neither shall thine eye pity him" (Deut. 13. 9.). The seducee is forbidden to state aught in his favor, as it is said: "Neither shalt thou spare" (Ibid.). If he knew aught about his guilt, he is not permitted to be silent about it, as it is said: "Neither shalt thou conceal him" (Ibid.). Whence do we deduct the prohibitive against a layman who is an enticer? From what it is said: "And all Israel shall hear, and fear" (Ibid. 13.12.)⁴Sifre Deut. 13; Sanhedrin, 63b. C.

A person whom he attempted to entice to idolatry shall not urge pleas for the acquittal of the enticer, as it is said, “Neither shalt thou spare” (ibid.).

A person whom he attempted to entice shall not refrain from giving evidence of the enticer’s guilt, if he has such evidence, as it is said, “Neither shalt thou conceal him” (ibid.).