(Spoilers All) So Varys also knew...

...about the Red Wedding before it happened. From one of Tyrion's chapters in ASoS.

At that Cersei lost patience. “It’s swords Joff needs, not toasts,” she snapped. “His realm is still plagued with would-be usurpers and self-styled kings.”

“But not for long, I think,” said Varys unctuously.

Edit: formatting