I think that she knew there was something going on way before Shaun entered the picture. I think she suspected that he was gay, but seeing him around Shaun, especially that day at the beach, with Cody, clenched it for her. You saw how angry Zach was at Shaun, she saw them at the beach (at the campfire)only a couple days before and they were fine and chummy. Only people who are "involved" act like they did. I love the stance at the end of the scene and the "Late." Next thing you know they are horizontal!!