It seems to me that she probably figured out what was really wrong with her marriage: the unwelcome presence of her idiot sister-in-law and her loathesome family. Whereever she turned her head, there was a Patterson in her face screwing up her life and bringing out the worst in her husband. Since she seems to be a fairly normal human being, it would probably have made sense to her to limit contact with the Pattersons so as to avoid unsolicited advice, annoying reminders of obsolete grudges and updates about the life of the scrawny irritant Phil was supposed to have done right by.
This, it seems to me, made it so very easy for her to convince him to pack up and leave for Montréal when her family needed her. He too seems to have finally realized that the best way to stay friends with Elly was to stay far away from her.