This is a tough nut, but in spite of identifying big-time with Nos's description (Mom's been a dedicated JW for 50 years this month, has been clinically depressed for at least the last 40 years and never sought treatment due to the WT's position on therapy)--I have to come down on the side of Mulan's prudent advice.
I've dealt with the honesty issue by simply responding to Mom's "What do you think?" sort of questions ("What do you think about the state of the world? Doesn't it look like this system is getting closer and closer to the end?") with this: "You don't really want to know what I think, because I don't agree with you at all on this topic. Let's not argue. Let's talk about something else."
Yes, I'm angry because of all she's given up to live in service to a book-publishing company, and I'm mad as hell that this sorry excuse for a religion hasn't given her any comfort at all, has in fact made her feel like somehow she's to blame for her depression and for the fact that none of her kids became JWs. But I also recognize that her belief system is so ingrained--and that the process of coming out of the Tower is so psychologically disruptive and painful--that forcing the issue might be too much for her to take. She's passively attempting suicide (refusing to make changes in her behavior and get medical treatment that could prolong her life)--I suspect if I made a concerted effort at this point to disrupt her belief system, she might actively attempt suicide.
It's frustrating. But I have to make a decision about whether a damaged relationship with my mother is better than none at all. I've had friends--and even a therapist--suggest that my mother is so "toxic," I'd be better off if I ended the relationship. That smacks of shunning. I want to be better than that. So I try to wage love, while keeping my boundaries as firmly in place as I can. Not perfect, but better than it might be.