First off I'm NOT bashing your girl's bike.
<rant>
One thing I never understood. Why people insist on buying bikes (and bike companies sell bikes) with "women's" step through frames instead of buying the appropriate size in a standard frame. In the 1900's, women's bike design was directed not by physical comfort, but by the enormous dresses and petticoats that were the standard at the time. The result was the traditional "step-through" or "Mixte" frame which has become synonymous with "women's bikes" and until recently was the only real attempt at women-specific frame design for nearly 100 years.
The average woman will need a bike with shorter overall "cockpit" length than the "standard" men's bike. Cockpit length is controlled by three things: handlebar stem length, seat position, and top tube length. (Two out of three are easy to adjust) Traditional "women's" or mixte frames have the shorter reach (effective top tube length) but are not as strong as the diamond frame, which makes me leery of them for offroad use
</rant>