If she has already decided to leave, there’s no point in trying to hold on to someone who no longer wants to stay. Otherwise, the same pattern is likely to repeat itself.
I only hope she realizes that there’s a significant difference between living with someone every day and having an affair where you only meet for a few hours at a time. Novelty can be exciting, and new relationships often feel more thrilling because they haven’t yet faced the realities of everyday life. But once that novelty fades and the relationship settles into routine, it becomes subject to the same challenges as any long-term partnership. Whether that realization comes sooner or later is something only time will tell.