What if you work at a public relations firm and the troubled website, Ashley Madison wants to hire you. (Ashley Madison is a website that matches married people so that they can have affairs) Can you take them on as a client even if what they do is unethical?
Please state the torah sources that give the answer to this question.
You should not take them on as a client.
What they do is not merely unethical but rather forbidden, for both Jews and non Jews alike. Hence even though the vast majority of users are non Jews, there would still be a violation of Lifnei Iver, to place a stumbling block to cause another to sin. This prohibition applies to non Jews as well, see Tractate Avoda Zara 6a, and see Shu”t Chasam Sofer C:M 185 who holds this is a Torah prohibition. Even if they could get this service elsewhere and is then not “lifnei iver” this is still a question of “aiding a sinner” which according to some opinions applies to a non Jew as well, see Rama Y:D 151:1. Aside from the potential above prohibitions, it would seem improper to be aiding and involved in such an immoral business in any way as this involves a person in a culture of sin and immorality, something a Torah observant Jew should distance himself from as much as possible.