You may not like this answer, but my response is there is no black and white answer. I would point out that looking for such an answer is only an indication of black and white thinking, and that never works well, I'm sure everyone can atleast see how things won't go over very smoothly with that approach.
As unhelpful as that may seem, I think it helps to explicitly state one other fact, and that is for the most part people do not want the truth about reality. Even outside the JWs people are deluding themselves in countless ways, and leaving the JWs does not automatically mean coming to the truth, or even having a better life, however you choose to define that. Just look at all the posts along the lines of "what do I believe in now..???"
Even if you care more about their freedom than risking your relationship with them, it's pretty obvious that such a thing needs to be done skillfully and with a certain amount of tact. I say just assess the situation from time to time and see how open they are, the choice is yours. Just don't confuse your own anxiety about them or even your own belief that someone should be told or not with how ready they actually are, which has to be based on what you can actually observe.