I didn't mean that you should have nothing at all to do with outsiders of that sort. Or with crooks, whether blue- or white-collar. Or with spiritual phonies, for that matter. You'd have to leave the world entirely to do that!
But I am saying that you shouldn't act as if everything is just fine when one of your Christian companions is promiscuous or crooked, is flip with God or rude to friends, gets drunk or becomes greedy and predatory. You can't just go along with this, treating it as acceptable behavior.