Edema! Edema that won't go away!!

Hello, My mother has Edema of the legas and ankles. She has had every test known to man including biopsy's etc. She is leaking fluid of the legs. This is scaring me deeply and can use any help I can get. She drinks Diet Coke like it's going out of style and doesn't excercise. She is not overweight, but I think her diet is full of sodium. She does have bug bites on her arms and leigns on her scalp now. Basically she is falling apart at the age of 64. I can't watch this go down like this anymore!! Please help!