It takes courage for a JW to read CoC.
As others have said, the person has to be willing to face unpleasant truths that will shake up his or her worldview.
There's no magic formula for knowing when a person is ready to do this.
Here are some diagnostic questions that MIGHT help get a JW to think (no guarantees):
1. "If the WTS is not what it claims to be, would you want to know it?"
Obviously, the answer to this question should be yes. If it is, then they should be willing to read CoC to find out what Ray has to say. But a JW who is honest may admit to himself that he really would NOT want to know. If he or she truly cares about the truth, this response should eat away at him.
2. "Other people -- Catholics, Baptists, Muslims, atheists, etc. -- believe they have the truth. In order for him to realize he doesn't have the truth but is on the wrong path, he would first have to be willing to question and examine what he currently believes. What would you say to him to persuade him to set aside his fears and make such an examination?" After he gives you the answer, ask him, "Why should a person do that when you aren't willing to set aside YOUR fears and examine what YOU believe?"
The goal of this question is to get the JW to see that he is constantly asking other people to do something that he is unwilling to do himself. If he is a person of integrity, this should eat away at him as well.
3. "If Jehovah wanted to tell you that the WTS is not what it claims to be, what would He have to do to get through to you?"
The goal of this question is to get the JW to see that if he is being misled, his refusal to check things out prevents himself from ever finding it out.
As I said, none of these questions is magical.
None of them is guaranteed to work.
For many, the idea that they may be wrong is just too painful or frightening to contemplate.