Is your mum on any water tablets - furosemide to be exact? This will help reduce the fluid in her legs. My Gran has a similar problem with her legs - they swell up enormously and hurt her very much. In my Grans case its because she has heart disease and kidney disease - fluid in the legs is a symptom of this. But I have found that when shes in bed, if i put a lot of pillows underneath her legs so her legs are higher than her heart - this seems to help.