She needs cushioning in the hip area.

If the issue is just at the hips, then I will join in the suggestion of adding extra padding directly under her hips in the form of a foam pad. I do something similar, putting my one foot square sitting pad underneath my Thermarest ProLite Plus pad right where my hips rest. For me it adds just enough extra to offset the extra pressure at that point.

This is about the simplest, cheapest and lightest solution possible, assuming you test it out and it works for her.

The suggestion to adjust the air so that the mattress is just soft enough that her hip almost touches the ground, but doesn't, is also sensible.

Good luck!