"I'm interested in skills to deal with people who mean well (or seem to), but ask questions that are none of their business either about me or about other people. Also, similarly, what do you do when your friend A asks you how friend B is? Suppose you've heard from friend B recently and you know they're not doing all that well, but you figure if they wanted friend A to know about it, they would've told them themselves. But there's no way to answer the question honestly."
When the question that's none of the questioner's business is about you yourself, and when your relationship with the questioner doesn't allow you to just Level and say, "I'm not going to answer that question, because it's none of your business," your best strategy is usually a Boring Baroque Response. There's a post on that topic at http://ozarque.livejournal.com/22528.html ; here's an excerpt:
Once in a while I find myself in a situation where I'm asked to teach just one emergency verbal self-defense technique and I'm given only fifteen minutes or so. When I agree to do that, I teach the Boring Baroque Response. It's an effective response to hostile or inappropriate utterances of many kinds. For example, if someone comes at you with "WHY do you eat SO MUCH JUNK food?" or "WHY IS it that you're STILL SINGle, at YOUR age?" (et cetera), you answer like this:
"I think it's because of something that happened to me when I was just a little kid. We were living in Detroit at the time ... no, wait a minute, it couldn't have been Detroit. It must have been when we were living in Indianapolis, because that was the summer when my Aunt Emily came to visit us, and she had one of those little yippy dogs, you know? And so..."
Long before you get to the first "And so..," your questioner will have said "Oh, never mind!" and gone away. No confrontation; no wasted time or energy; no loss of face on either side.
When the inappropriate question is about somebody else, you need a different strategy. I would suggest that you just Level and say something along these lines:
"It would be best for you to ask [X] about that directly."
"It wouldn't be appropriate for me to answer that question; you need to ask [X]."
"I wouldn't feel comfortable answering that question; you need to ask [X] directly."
If the questioner is someone who drastically outranks you, you may not feel that the Leveling responses are safe. In that case, as always, you should go to Computer Mode, as in this dialogue.
BOSS: "What's WRONG with Tom Cartwright that's making him turn his work in LATE all the time?"
JOHN: "It's always hard to figure out the reason for someone else's behavior."
BOSS: "Well, it's not LIKE him. He's always been reliable in the past."
JOHN: "A lot of different things could explain why someone's behavior changes."
BOSS: "Well, DO YOU OR DON'T you know what's wrong with him?"
JOHN: "It's easy to make a guess about someone else's behavior; it's also easy to be completely wrong."
--OR-- "Relying on a broken crystal ball is a good way to end up with bad data."
--OR-- "Work turned in late is always a problem in the workplace."
And so on, for as long as it takes. The boss may not be pleased with John's responses, all of which make it very clear that John's not willing to answer the question posed -- but they're not confrontational, and they're not openly rude. If John is really determined not to betray Tom's confidence, and it's not possible to Level with the boss, they're the only alternative.
When the inappropriate utterance isn't in the form of a question -- as in "You know what's wrong with you? YOU eat TOO MUCH JUNK food!" -- you can modify the Boring Baroque Respose and do it this way:
"Hearing you say that reminds me of something that happened to me when I was just a little kid...." Or "Hearing you say that reminds me of an article I read only the other day..." or "Hearing you say that reminds me of a dream I had just the other night..." followed in each case by interminable irrelevant boring details.