Ex-etiquette: Who is the co-parent?

A parent is experiencing a shift in co-parenting dynamics as their ex-partner's girlfriend has begun managing communications regarding their children. The girlfriend handles various logistics, including pickup times, homework, and schedule changes. While the parent describes her as pleasant and expresses no personal dislike toward her, they are questioning why they are effectively co-parenting with the girlfriend instead of their ex-partner, raising concerns about the appropriate boundaries and roles within their family structure.