If you have time to prepare, read the following books in this order:
1. Steven Hassan's Combatting Cult Mind Control (teaches you the proper WAY of approaching someone in a cult. Not WHAT to say so much as HOW to say it, with body language, friendship, words to use and not use, and preparing friends and family who might be involved. It is NOT RECOMMENDED to skip this step!)
2. Read Thus Saith Jehovah's Witnesses and possibly get the Document Pack for the same book, if you are going to be using photocopies to prove your point (don't show them the book). This will demonstrate their dishonesty and false prophecies, changes, and techniques to control people they have used for the last 126 years. It is to be read from page one straight to the back in an hour or so. If you want to get the Document Pack for showing them what you have found, they are loose sheet photocopies without commentary (the Thus Saith book has commentary). Don't get the Document Pack only, as you won't know how to use it unless you are already good at this, which you wouldn't be if you are reading this.
4. Optional but very helpful books are:
Crisis of Conscience by Raymond Franz
NOW ON TO TECHNIQUE
IMPORTANT - Stay away from doctrinal discussions such as Trinity, hell, going to war, etc. This is their playground and they are well-trained to develop "alternate realities" as to what scriptural passages might mean, or how they COULD BE interpreted, as opposed to how they were HISTORICALLY interpreted.
Approach it from: "If I were to let any organization (Scientology, Moonies, etc.) teach me doctrine or interpretations of the Bible, I would first want to know that I could TRUST such an organization. What does their record show about honesty, consistency, and how well does the pattern of life in the organization match the early Christians? I would also want to know what their detractors say about them, because that often gives away a lot more than what the group says about itself!" So that sets the stage for discussing history and mind control techniques, not doctrine. Only if we can see the organization is honest and like Christ can we proceed with doctrine. Otherwise the clever arguments of men might fool us.
Here is the first info to memorize and use, "Opening the Closed Mind": http://www.freeminds.org/psych/openmind.htm
As far as honesty in doctrine and being a false prophet: http://www.freeminds.org/history/part1.htm
Questions you ask before you join a group: http://www.freeminds.org/psych/consumer.htm
On the existence of Thought Reform and why you are leery: http://www.factnet.org/Thought_Reform_Exists.htm
The Christian VS Watchtower way of raising the child of God: http://www.freeminds.org/doctrine/rules.htm
Other helpful tomes can be found HERE.
Try to devote the first session to only one subject, using the "Opening The Closed Mind" material. If you cannot "get their goat" using that, it is unlikely you will be successful with other approaches. Do a little research on early Christianity in the library if you can, like reading about the early Church Fathers from 100-300AD. These are the ones who helped in the canonization of Scripture (deciding which books would later become the "Bible.") Obviously they were the first known interpreters, not Jehovah's Witnesses. My favorite book is "The Canon of the New Testament: Its Origin, Development, and Significance" by Bruce M. Metzger (Oxford University Press; 1987; ISBN 0-19-826180-2).
The goal is to get their inner mind to question what they have been taught by their leaders, the "faithful and discreet slave" (i.e., the Governing Body). If you see that you might have done that ever so slightly in the first session, arrange to further discuss the idea as noted above,
"If I were to let any organization (Scientology, Moonies, etc.) teach me doctrine or interpretations of the Bible, I would first want to know that I could TRUST such an organization. What does their record show about honesty, consistency, and how well does the pattern of life in the organization match the early Christians? I would also want to know what their detractors say about them, because that often gives away a lot more than what the group says about itself!"
This can be done by utilizing material from Thus Saith Jehovah's Witnesses, and it would be good to have the photocopies ready (Document Pack or other similar collection). Try to memorize how the Thus Saith book develops a point in each chapter, and you can even put your copies in order as the book uses them for help in remembering where you are in the discussion.
Save the information about the Watchtower joining the United Nations as an NGO (last section of Thus Saith book, also here) for the final blow if you can. Make the point that, "It seems to me that the Watchtower organization has apostasized from their own teachings regarding taking the Mark of the Beast!" It is a very serious matter to take the hand of the devil after years of fighting him."
If you have managed to get this far and have aroused even the slightest of doubts in them as to their trust in the Governing Body, it may be time to go on to Scriptural subjects. Allow them to bring up one that most concerns them, then look it up in the book Refuting Jehovah's Witnesses (such as the Cross, the name Jehovah, Jesus' nature, Holidays, etc.) After you cover one subject in depth, they will see that there may be more issues that they have not learned properly. Take it slow, give them time to let it all sink in. Destroying one's cherished beliefs is not to be taken lightly, as they may become very uneasy and insecure at this point. Be a kind friend to them, but not phony. By now they will know if they can trust you or not. Be there to help them on the road to recovery. When they finally ask, "Where else can I go?" THEN is the time to let the Holy Spirit come in and lead you to read from the Bible itself. I like to start with favorites such as John 3:3-8; John 6:35-40, 44-58; John 7:38. John 14:1-3, 14:15-21. Pray and let the Spirit lead you. If the person is so inclined by the Spirit to push farther, let the Spirit guide you both. If not, or not at this time, you have not failed. You have helped set their mind free of cultic control, and they are free to pursue life as they see fit. Many come around to spirituality in time, don't try and lead their life from that point on.