Originally Posted by Wraith Rider
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.
This is the point that everyone else seems to have missed. If your bike will do a "stoppie," as most bikes will, then you can get 100% of your braking power out of your front tire. It's just a matter of learning where that threshold is and being able to stay just under it reliably.
Until your girlfriend becomes an expert at stoppies though, she should be advised to practice emergency stops using both brakes.