The senior, the training captain, will not feel any inhibition in speaking up when or if he observes the junior, the trainee and the PF, doing something stupid.
Yet he did not do so in this situation. He let the trainee crash the aircraft.
Being an instructor does not automatically put him above the trainee. If, for instance, both of them have a military background, and the trainee held a higher rank back then, it could affect the dynamics in this cockpit.