Got an interesting question the other day: “If the heroine ends up with her ex-boyfriend’s best friend, does that discount the story as a romance?”
The short is “no,” the longer answer is “it depends.” The relationship dynamic can fit romance if you start the book right after the break up. She’s packing up her things, giving the ring back, whatever. That shouldn’t take up too much room. The crux of a romance story is to explore the courtship and union of two people, so the vast majority of the story should be about her and the ex’s best friend.
If you spend a lot of time on the past relationship,then the breakup, then the new relationship, then your focus is really about her personal journey of transformation, which in my mind, would make it women’s fiction.