The front brake supplies 70% of the stopping power. The rear takes up the slack. Why only use 70%(probably less if your not using bot brakes) of your stopping power? Learn/teach to use both brakes...and use BOTH of them. Practice your "emergency" braking technique often, BEFORE you need to use it.
What if your girl friend has an actual emergency braking situation, left hand turner, deer runs across road, etc? With out ever using the rear brake or practicing any kind of emergency braking the scenario will most likely not turn out well.
If you are worried about her locking up the rear tire...practice,, practice, practice, makes perfect, or close to it.