I'm afraid that VM44 speaks the truth, if this guy who is an ordanded minster in another chruch startes to make progress the JW will just stop talking to him. If there was an easy way to convince people that this was a cult everyone would be out already.
Arguing with a JW
How about these:
"Hey, I hear that you guys excommunicate for smoking, and that you are not even allowed to talk to the person afterwards. Is that true?"
"Do you personally know anyone who has been excommunicated? In your immediate family? When is the last time you talked to them?"
"I hear you won't take a blood transfusion to save a life. Have the rules changed over what you can and cannot take over time?"
"Is it true at one time Witnesses couldn't take organs, as it was considered cannibalism, but now you can? Would you accept a transsplant? Would you be excommunicated if you took a transplant? How about before the policy change? How can you deny someone salvation based on a policy change?"
"Do you have any family that aren't Witnesses? When is the last time you visited? Why?"
"Do you consider me bad association? Why? Do you believe we worship the same God? Do you believe there is only one God? Well, then, if you don't think I worship the same God as you, who do you think I worship, Satan?"
"How about your charity and good works?"
"That's nice that you help each other. How about people like me, who you think is worshippin Satan? Would you offer any practical helps beyond a magazine placement? How many magazines have you placed this month? How many of the elderly or sick have you visited?"
Try hitting them with their failure for compassion, rather than doctrine or scandal.
Certainly "arguing" will get nowhere. Maybe asking a few pertinent questions from time to time will open the JWs eyes over time.
There is a lot of info at www.jwfacts.com. Different aspects get through to different people. Some are shocked by the UN association, some by the latest pedophile settlements, 607, or the history of prophetic failure. Doctrine rarely works, but things like how Jehovah was added to the NT can have an affect. Your friend needs to touch on each of these topics and see which ones have an affect on the JW. One thing to do is ask why he thinks he has the truth. Often this will uncover the area that will be most important and hence undermining that particular reasoning will be most productive.