There's possibly a buried lede there, of the GF feeling like she cannot ask questions for clarifications or cannot let the commenter down.

Or infinite reasons:

- Rain distribution isn't uniform, it poured where she was and thought it was necessary.

And/or

- Overestimating time/sunk cost fallacy ("it's just going to take 2 minutes")

And/or

- Giving less importance to ponctuality than others

And/or

- She didn't have one and thought it was a good idea, worth being late

And/or ...

GP expressed the need for an umbrella and she went above and beyond. He now knows that she's the type of person who will do so, so now knows to add a disclaimer to not go out of her way. Or he can just appreciate the fact that she will go out of her way and sincerely express that appreciation.

No fault here for either party. Just a learning experience. And as you say, he can give the feedback that she didn't need to go out of her way as well.

To me the premise "if you haven't left yet" implies "don't go out of your way" and adding that would be redundant. But not everyone sees things the same way so you have to adapt communication individually based on experience.

Exactly, I write a literal message with a literal if/then meaning. She reads it as "my partner is out in the rain and would like me to help".

Of course I think _my_ communication style is "better". But the best communication style is actually the one that communicates your thoughts to the other person, not the one that "makes the most sense".

And meanwhile there are still cases where it will be the other way around and I am going "well I said X, but surely you could have read between the lines..."

So yeah, the best response is to tell her about the miscommunication so she can know better next time, but it's stupid to try and say she was "wrong". It takes two!

It's not about 'fault' really, it's about achieving better outcomes by learning from previous experiences.