I've never been entirely sure about the "about something other than a man" part of that. Say the heroine and her female mentor discuss the despicable murder of her sidekick by the villain -- does it really make the story less woman-friendly if villain or victim is a man?

I'd have thought not, honestly. But then I'm (obviously) not a woman.

It's also a difficult rule to apply consistently to SF, where characters might be hermaphrodites, sex-changers, shape-shifters, androgynes, robots, AIs, creatures of indeterminate gender, triple-sexed aliens etc. In some contexts, introducing women would in itself be a retrograde step towards familiar gender categories.