What it takes to be nosy

Lately, I have been exposed by a friend's stories, about people around her. She can talk for hours when venting what she has in mind about those people. Apparently, the things she talks are more on her dissatisfaction about them. And apparently, I know some of them. Though I disagree to some parts of her stories, I have been trying to be a good listener by refraining myself from venting my disagreement. Maybe she just needs someone to listen to her right now. She is a nice person, actually. Only lately, maybe, she's not in the right state to be nice...I guess.
Sometimes, if She starts to become judgemental or cynical (or even bitter), I would cut in to stop her. However, I never thought that her 'stories' will go far... so far that she even evaluates people who do not have anything to do with her, but have things to do with me.

As far as i remember, i never asked for her opinion about these people under my responsibility. And honestly, i don't understand why she would like to talk about these people and judge them this and that. What is it with her?

I've tried to 'softly' remind her that it's not her domain to evaluate these people. She sometimes starts to be aware when my speech tone raised.

I am wondering what is actually happening to her. Coz one person she has been dissatisfied about (and apparently someone i know) came to me and told me that she's been treating her not nicely and her mood is not as usual.
Though I start to feel irritated by her being nosy, I hope she will get back to normal soon.