Scoliosis made worse by memory foam, can you help?

Hello,
my sister needs a new mattress, currently sleeping on memory foam. She is handicapped, 4'8" and weighs approx 12 stone (increase in wt over the last year). She has scoliosis and is unable to adjust her position in bed, meaning she sleeps on the same side all night. She is troubled with a sore hip during the night and wakes up with a sore side, although there is no indication of pressure sores. She did have a pressure sore, which has cleared, but which has left her side tender. Could you recommend a mattress which could help alleviate her pain.
Thank you
Doug