My mom is paraplegic and has had a large pressure sore on her lower back and buttocks for nearly a month now?

Pressure ulcer. Your mother should be seen by a specialist in wounds, either home health nursing or in a wound center if the travel is not too onerous. There are several changes that you will need to make in the home related to equipment (mattresses and cushions) and activity and turning schedules. You will also need to have a nutritional consult also related to protein intake.
Air matress. She must use air mattress and positional rotation each 2 hours to avoid ulcers...!