pain when sitting

My friend has pain when she sits. She has been told that she has arthritis in her spine. She has been to several specialist with this but no one thinks it is her coccyx because of no pain when they manipulate; however, when she sits she has to sit on a pillow, ice, and cannot wear anything that touches it. It seems seems to radiate into her left leg. She has had sciatica in the past also. She has no pain in her coccyx area when standing. Do you have any idea what could be wrong? I seems to be a nerve thing due to her description of the pain; burning sensation, etc.