How do you separate a narcissist from his willing victim?

The question I have for you is why would you want to do this? I understand that you think that this narcissist is bad news, but your question specifies the other person involved is "willing". Perhaps you do not understand the dynamics of the relationship properly. Maybe you are meddling where you shouldn't. Take a look at your reasons for doing this before proceeding. I understand what you are asking since I used to be one of those "willing" victims who was dazzled by the narcissist's charm. Lots of people did try to warn me that I was being used, but I was too much in the thrall of the narcissist. It took 8 years and 2 divorces before I came to my senses. Sadly I don't think you yourself can save the victim of a narcissist. The victim has to save themselves. You can tell them what you know during a time when they seem willing to listen, and hope they will eventually save themselves. Most do eventually get out. It is easy to get away from a narcissist once the charm wears thin and you finally see through them. The more frequently the victims hear the truth from others, the more likely they will get out sooner. When they finally do get out, they will be a lot wiser. Personal boundaries might be a good neutral topic to discuss with the victim of a narcissist, since that is one area where a narcissist runs rampant over his victim. If the victim realized how much his/her personal boundaries are being ignored, the victim could begin to put two and two together.