I've dated, and been close friends with, a few "free spirits" and noticed this same "pulling away" behavior you describe. For me, talking to them about it or trying to negotiate never really worked. They will either avoid the conversation, deny they're doing anything differently, or try to blame you. Usually your only choices are either to accept the situation as-is or to end it, either by pulling away yourself or by explicitly breaking up.
I would encourage Z to take back power and made a decision how to move forward, one that is not depending on or waiting on this other woman's behavior, which is out of your control.