I am in this exact situation with my aged father. To be honest if he woke up, at his age, it would certainly kill him and I would deeply regret being the cause of this. This has also caused a bit of a love hate relationship with Wt. Don’t get me wrong, nothing would bring me more satisfaction to see the Tower burn to the ground. Then on the other hand I have to be pragmatic, in that it’s a support net work (be it conditional), and it occupies his day. If he was to walk out of the religion at his age he would be left with nothing. So in his case he is better off left ignorant, and I choose to keep him that way by keeping my mouth shut.
Now, on the other hand, if I was approached by my teenage JW relatives. I would certainly not hold back on telling them the facts. The are healthy enough and adaptable enough to make the shift into secular society. But I would certainly want to warn them of the dangers as well e.g unsavoury individuals, addiction, healthy sexual practice. So I guess weather or not to wake someone up can be a cruelty or a kindness.