How much information is one obligated to share about others?
I have found myself recently in a number of strange, not at all life threatening but still annoying situations, which made me wonder whether to disclose information. Usually, I don't gossip much, but do I have an ethical obligation in any of these situations, or ones like them?
Or is intervening simply none of my business?

1. A kind woman of my acquaintance, "Y", who runs a business is considering hiring "B", a former boyfriend of a friend of mine as her delivery man. I think he is not harmful, but is a bit of a liar and not a great person, but this is because I saw him go through a bad relationship with my friend.  Y does not know I know B and did not ask my opinion of him. It is however a small business, so she is likely to see a lot of him. She is also single, is likely to be attracted to him and that is likely to end badly. You see? I'm not judgmental at all. :) But I would feel oddly guilty if this happened.

2. A co-worker of mine often brings baked goods to share at work, which is, for her, important emotionally as it makes her feel very appreciated. I happen to know, because she casually mentioned it, that she does most of her cooking at a collective kitchen where she volunteers. I know the place, and the sanitary conditions there are not the best, let's put it that way. It makes me a little bit queasy, but is it really my duty to alert others? I have no reason to want to hurt her standing. And everyone eats in all sorts of restaurants anyway, who knows what goes on there.

Well, you get the idea from these examples. How do you deal with these sorts of situations?