Post slut syndrome. My ex fiance tried to rekindle things years after we split, and she "found herself." Welp, I needed to "find myself" too. And I found myself someone leaps and bounds beyond anything she could have ever been. Maybe I should send her a thank you card?
Side piece is probably an upgrade in all areas. Good for him. Hopefully he does enter into a monogamous relationship, and turns that side chick into his one and only.