I'm listening to The Gathering Storm on CD and Rand is now in Bandar Eban after exiling Cads. When Nyneave is attempting to figure out how to reason with him, she muses that the only Aes Sedai he somewhat trusted was Moiraine because of what happened after they went with the Aiel. How did Nyneave actually know that happened? She wasn't there.

 

So is this a screw up or did Nyneave hear about it and I missed that or does Nyneave know to advance the plot?

Egwene had meetings with Elayne & Nynaeve in Telaranrhiod. Shadow Rising & Fires of Heaven

In those meetings, each told what was happening in their area.

And Nynaeve actually had a moment where she silently berated herself for defending Moiraine.