Don't use the "c" word ("cult") with them. Even if they use it, don't use it yourself. They will inevitably use the word with their parents in the following context: "JWs are a cult and even Uncle CoCo thinks so too!" When that happens, the iron curtain will be raised and your access to dear young ones will be eliminated.
Encourage them to broaden their horizons. There are books and movies that they can read or watch that will assist them to make a cognitive connection that the JWs are not a socially, mentally, or emotionally healthy group. You can offer to take them to a play, like Man of La Mancha, or buy them a copy of Orwell's 1984 or Animal Farm, or even The Red Tent by Anita Diamant. Talk to them about the oppressors (antagonists) and the freedom fighters (protagonists) in these works, and the kinds of tactics that were used to control people. Do not make mention of the JWs. Teens are bright enough to make the connection without prodding.
Encourage them to bide their time, be respectful of their parents, and think about their future. Getting out of the JWs is going to require that they finish school, probably pursue higher education and maybe even leave the nest sooner than they were planning to do in order to make their plans come to fruition. They may need to think about getting a part-time job after school so they can save money. They may need to have a safe haven if they find themselves on the cold side of the WTS's "love". Let them know that if they find themselves in that situation, that you will help them to the best of your ability.
I really wish I'd had someone like that when I was 16 and knew in my heart that the JWs were a cult. It would have been much harder for them to suck me back in to their vortex.
