I think ‘childish’ simply means: it’s complex and you don’t understand.
OK. Dangerous tactic here, and we may have to start calling you ‘One Eared Funk’ from after, but why not just stand up to it a bit? I don’t suffer from (those) ladies problems nowadays, but I know that restless, frustrated, everything gets on my nerves feeling. I definitely start it more if there’s an easy target with no consequences for me. You don’t have to be aggressive or mean. Just say ‘be unhappy then: there’s clearly nothing I can do to help. I’m off to enjoy my weekend.’. She’ll break stuff the first time you do it, but at some point she’ll just start to chill out and work out ways to deal with it.
At the moment, you’re validating it by getting involved (staying to listen is getting involved).
She may be having problems expressing herself appropriately/assertively in other areas of her life, so it’s easier to externalise the bad feelings she has as a symptom. Is her boss a bully and she can’t respond to him? Maybe her parents? Maybe she needs confidence to be more assertive elsewhere. Obviously that doesn’t develop overnight, but maybe you can encourage her to express herself more? You could look on it as a compliment that she feels safe enough to push it with you.
Tell her how you feel (on days 10-20 of her cycle…)
Sorry if this sounds patronising or doesn’t fit the situ.