she Really misled him
Watching the movie a third time, I paid close attention the married woman pretending to be single. What a horrible way to treat someone. I can understand the "just having fun" part, but when he invited her to meet his family (at the wedding) she should have come clean: "Look this is getting too serious. I was just looking for casual sex."
Instead she pretended to be interested in solidifying the relationship... a horribly mean thing to do
.