So do you think the answer is automatically divorce for every single instance of flirting? Is that realistic?
I personally don't, no, not on an instance of flirting.
However, if my partner was having sexually charged conversations (especially on an ongoing basis like this - not a one off mistake) with an ex (or anyone) then yes I would personally break up.
Nobody has said it's simple or automatic, you're making it sound as if they have but having a clear boundary and setting a bar doesn't mean it's easy to leave.
It's awful and upsetting and takes its toll on someone to break up with a partner who has hurt them. It doesn't mean it's the wrong thing to do.
You are of course entitled to your opinion, if you'd stay with someone who was talking to someone they used to sleep with about how great it would be to fuck them again and date them again then that's fine.
It's just not a typical bar of what's acceptable in an exclusive relationship.
FWIW I agree with you regarding snooping, it's wrong and a huge invasion of privacy, and in my opinion only happens in relationships lacking in trust already. But the OP's mate has done it now and so people are responding based on what she already knows.