I liken reading about a third party to talking about a third party. Would I talk about someone's relationship, mental status, life situation normally? If no, I won't read about it either.
When put like that, you can see that I think it's not black or white. If I had a friend who was trying to move on from a breakup and they wanted to talk about their ex and their current relationship status as a way to process their feelings, a way to let go, then I'd definitely talk with them about it and may even read about it for them. If my friend was self-flagellating, I wouldn't want to be complicit in that, so I wouldn't do either in that case.
If I didn't know the person asking, I'd try my best to apply the above to them, keeping in mind that we're all adults here and I'm not their mama.
Is talking about someone without their knowledge an invasion of privacy? Unless you're naming names while outing all sorts of private information, I don't think so. However, others feel quite differently, so I doubt I'd go on an open forum and discuss having done it.