The correct answer is: Lochinvar was chivalrous and dashing--when he walked in to Ellen’s wedding—and swept her away.
Indeed, the core meaning of the sentence is that Lochinvar swept her away because he was chivalrous and dashing. However, it does not mention when that happened; and the author has chosen the dash in order to provide further explanation of the context. Because such piece of information is not necessary for the reader to understand the overall sense of the main sentence, the correct use should be the one used in the selected sentence since it isolates the piece of additional information in the middle of the main sentence.
