They are usually caused by not turning. Air mattresses may help. The lower back are the worst because there is hardly any tissue to help in healing. Once you have one, should be taken care by a wound care nurse. A lay person may not know the signs of dying tissue.
Make sure they eat enough protein, like eggs, meat etc. never keep a dirty diaper on for more than 15 minutes, use a barrier ointment like A&D. Dont stay in the same position more than 2 hours. Sitting in poop or urine can really destroy skin.
Can she:
walk
stand
sit up in bed
position herself in bed
roll from side to side
and do any of the above with or without assistance?
https://www.agingcare.com/topics/235/bed-sores