I'm really not following your logic that she shouldn't be learning how to use that rear brake in "NEED TO STOP NOW" situations because she may be panicking and over do it.
How do you think she's going to develop the conditioned response (uh oh, I just used a psychological term) required to not lock up that brake in an emergency situation? She has to practice using the rear brake in a firm, progressive manner.
I know other people have said this, but I figured it needed to be repeated one more time for good measure.
In emergency situations you want to stack the cards as heavily in your favor as you can.
I ride 805ccs of slug.