I think you do know what to do; you're just conflicted about what will happen to this person. There isn't anything I can say that hasn't been voiced by everyone else here. Do the right thing.

Put yourself in their shoes.... wouldn't you want someone to step up and help you? And if this so called "friend" has no qualms about forcing his employees to sleep with him who knows what else he is guilty of?? What's to stop him from doing something to you or anyone else? What happens to him is none of your concern because he belongs in prison.