What does Irene do that causes Bill to make a scene at dress rehearsal in Atlantic City in the novel, Two Wrongs?

When Bill’s play opens in Atlantic City, his fiancée Irene Rikker “had come down with Frank Llewellen on a different train,” allowing herself to be publicly seen going along with Frank instead of Bill. The train ride is the explicit reason why Bill causes a scene during dress rehearsal, and it symbolizes the dissolution of their engagement.