In Australia try CAR 226.
CAR 226 Dual controls
(1) During flight, a person may occupy a control seat of an aircraft equipped with fully or partially functioning dual controls only if:
(a) the person holds an appropriate pilot licence for the type of aircraft and the class of operations in which the aircraft is flown; or
(b) the person is a student pilot assigned for instruction in the aircraft; or
(c) the person is authorised by CASA.
(2) In authorising a person to occupy a control seat in pursuance of subregulation (1), CASA may grant the authority subject to such conditions as CASA considers necessary in the interests of safety.
Had a cabin crew member sitting in a control seat once on return from the bathroom. Politely informed them that they were breaking the law and not to do it again as the next person may not be so forgiving. Cabin crew are not authorised to occupy a control seat.
As an exercise, ask the cabin crew what they think the purpose of them being on the flight deck whilst the other pilot uses the bathroom is. You may be surprised to find a variety of answers.