Both DH and I have had accidents in the past with damage to our spines, resulting in arthritic and nerve pain getting worse as we get older. We bought a memory foam mattress, in Portugal, which was a great relief at first but as the temperatures warmed up I (not DH) started having problems in my feet, supposedly due to spinal nerve damage. We cooled the mattress down a lot by buying a quilted cotton mattress topper and my feet got a lot better.
I think some problems are made worse by poor circulation, which possibly your MiL being female, unwell, and not being able to move around suffers from.
When it gets really hot in Summer I think about investing in silk pillow cases, maybe sheets, which are supposed to be cooler.