Reach too far stretching her out. Causing a pinch at the hinge point. Effectivlely making the hips rotated too far back by means of reach.
Reach too high shifting weight to back, butt, sit bones, compressing her spine/lower back.
Missing core strength to hold it all together.
All solid suggestions.
Not mentioned yet: Stretching to incorporate & activate the necessary muscles in order to better share the load/stress/strain of the activity.
In short, I agree with the posters here. Lower, closer bars and exercises. Professional fit.
...Or, she could just buy a 3 speed Dutchie intended for 3 or 4 mile trips to the grocery store.