A man jumped to her defense, which was sexism, and then her co-workers got resentful, which she wanted to be defended from but wasn't, and that was sexism too

Being a privileged, white male it was hard for me to see what woman are put through in terms of casual sexism in the workplace and in society.

Since my girlfriend works as a secondary school teacher I see much clearer what's wrong and it's partially very subtle.

Here's an example: performance review? Criticism about how she dresses. This would never come up with one her male colleagues (and yes, she checked).

That's totally casual sexism over which you may "harumph!" about. But unless you're a woman (or another discriminated against minority) you're in no position to make that determination.