<human>: I work at a fairly small company, around 20 employees. It's owned solely by our CEO and is a nice place to work.

The finance manager here is kinda ... gossipy. And bitchy, maybe. We're not close, not really friends. We sit on opposite ends of the office. She's ok, but we don't hang out outside of work. I'm not her closest friend at work, she has people she sits with and goes out for lunch with every day. So if she's said something to me, she's probably mentioned it to others.

Last week she ranted to me a bit about her salary. She told me she makes 45k GBP, which she doesn't think is enough. OK. I don't need to know this. Only she THEN tells me how much the CEO makes. She literally tells me the exact details of my bosses take-home pay. She also goes into details of another director's salary.
I am very, very uncomfortable. I've done some googling, and I think this could even be illegal (?). That could be wrong though.
Exactly how bad is this, and what will happen if I tell the CEO? Do I have to tell the CEO? I really don't want to get anyone fired or be called a troublemaker or gossip :(

Our owner is a really nice woman, she's always been very kind to all her employees. I work under her directly and get along with her well, but she's still my boss. I don't even know how to bring this up, or if I'm blowing it out of proportion completely.

Give me the gist of the text in a nutshell
<bot>: finance manager told me how much my boss makes. Suspect she's told others. Do I tell my boss? How to avoid looking like a troublemaker?