There will never be a right answer for that question.
Tell the beginner to concentrate on best braking with the front brake, ignore the rear brake?
Tell her to concentrate on the front an just lock up the rear, because it's controllable?
Tell her to brake as hard as she can with both brakes, because most beginners not even nearly use the full potential of their brakes?
Every one of these advices is reasonable or not, depending on whom you are talking to. You have to know the person.
On my bike the front brake only does about 85% of the job. I nearly allways use both brakes, no matter what, but then... I have C-ABS and don't have to worry about locking up anything. On a sports bike under good conditions you might ignore the rear brake because you will lift it, on the other hand if you don't (nearly) lift it, there WILL be some traction left on the rear you could use to brake.
I'd go with the advice to allways use both brakes and IF the rear locks up, leave it locked up for the beginners or release it if you have quick reactions and some training.