That is funny you ask for opinions then argue with anyone with a different view than yours.
If you're worried about her locking the back brake by just stomping on it what makes you think she will be any different with the front? You don't think she will just grab the hell out of the front and go down when it locks and washes out?
Personally I think it's easier to recover from a rear brake slide than a front one, even for a beginner. All they have to do is keep stomping on that pedal.
What scenarios do you anticipate her making a hard stop? (I don't think she is doing stoppies, so the rear wheel coming up shouldn't be factored in. I would think she locks the front wheel before it comes up)
Straight dry road - using both brakes and locking up the back should be no problem
In a turn - grabbing the front usually makes you go down, stomping on the back makes you fly then hit the ground, using some of both works
Slick road - grabbing the front not good, stomping on the back not good but probably not as bad, using both good
Also, I guess a lot of this depends on what kind of bike she rides.
Practice using both would be the best as others have said.