Obviously, the OP is inflight crew. As such, writing up or reporting to management another FA for something short of putting the crew, plane, or passengers at risk is considered in very bad form and risks the on-board relationship with other crew.
However, there certainly has to be a way to report this departure from the book to an inflight supervisor who can remind these particular FAs that creating one's own rules is not acceptable and they need to get back on track.