hi,
how are her biomechanics during sitting/standing? observe whether she has abdominal muscle/quads, hamstrings/gluts/erector spinae etc muscle imbalance (yanda). also observe relative strength of the muscles which help maintain posture does she lumbar flex or extend during sitting, does lumbar support or lack of lumbar support help? does changing the tilt of her pelvis assist with sitting? is she rotated during sitting?

the idea is that if she's getting claudication during sitting, find out whether it's due to the position/ relative flex/ext of the spine that is causing the claudication and whether you can perform exercises to help strengthen the relative weaknesses to allow her to maintain a good lumbar position that she can feel comfortable with.