Let's consider this from the game shop or convention setting and the author's point that people need to stand up and say something when a group member misbehaves.
A good idea.
So we're at the game shop, you (any reader, not just Umbran), me and Danny are playing a game. A woman walks in and I say something crude involving her and my THAC0 that could be considered offensive, sexist and just wrong on non-euclidean geometry levels.
What can you and should you do (in a non-violent, legal response)?
One of the things to consider in this example, is that YOU are not an authority figure, you are just a customer like me. But you are a Third Party, per Umbran's framing.
Well, in a typical RPG, there is a sort of authority figure - the GM.
As a GM, the response is pretty simple, to the effect of, "Dude, that was *not* cool. Maybe I didn't make it clear, but I have a strict, 'No jerks,' policy at my table. Apologize to the lady, or please pack up and go." This all dependent on what the store or con's policy, if any, may be.
As a player, to the effect of "Dude, that was *not* cool!" To the woman: "I'm sorry you were subjected to that. It wasn't acceptable behavior." To the GM, if any, "You going to allow that kind of thing at your table?"