+1 for coconut fibre mattress. You may feel it is a bit firm, but it does wonders for your back. Really makes a huge difference.

As an older person, normally we want medium firm - firm mattresses. Avoid the soft mattresses.

However it also depends on your mum's personal preference as well as how severe the curve is. 100% coconut fibre isn't really good if the curve is severe, so you may want to look at the hybrids. My friend has a hunch over, and he uses some pillows to help support.

If possible, maybe can buy one hybrid mattress and ask her to try? If it doesn't work then go for something firmer or softer.