وعليكم السلام ورحمة الله وبركاته I advice him to reject any request from those kind of people since they don't deserve any treatment and it is a way to show the anger from a believer towards them. But if he chooses to deal with them then it doesn't mean that he is earning provision in a Haram way. And the same applies to serving anyone who is doing Mujahara with their sins (i.e those practice sins openly). And Allah عز وجل knows best.