So... like 75% of bike riders should not be allowed to ride their bikes in the rain? Perhaps you should come give a bike safety seminar at my school.

It is also not that she can't operate a bike with only a front brake. It's that she herself does not feel safe bombing down a hill with no control over her rear wheel. When you go down a hill, your weight is already shifted to the front of the bike. Braking with only a front brake also shifts your weight to the front of the bike. I think you're smart enough to put these two together.