You wouldn't be able to prove to any Jehovah's Witness anything you just plant seeds and hope for the best,, to a long time member close to or in their retirement years I wouldn't usually try, why pull the rug from under them when they are so close to the end of their life/membership to the corporation? It all depends on what good would it do in the final out come.
But if an elderly JW was to ask me question my replies would be(I hope) with his best interest. I'm all for a gentle removal of JWs from the cult, and the eventual demise of this evil mind controlling cult, but people are people and we should care what's going to happen to those JWs that we help awaken from the Watchtower dream/utopia. Just plant seeds and let nature take it's course and enjoy your new found freedom from these dumb bastards.