She was honest all the way through the breakup, and his hurt feelings were efinitely his own doing
But, things change at the wedding, where they dance and share an intimate moment. She knows how he feels about her, and she invites him to her party without ever telling him it is an engagement party. She totally fucked him up on that, and it was a horrible thing for her to do. She needed to tell him beforehand.
Still a wonderful movie. It's not as good as Eternal Sunshine, though I'm open to that being a GenerationX versus Generation whatever it is now.