How do i get help for my nan to have a knee replacement?

My nan is overweight, she has had bad knees for years, she was promised a knee replacement but the anisatist would not put her under becasue she is fat. She cannot loose weight because she cant move or do any exercise. She really doesnt want to be bed rest for the rest of her life so she really needs her knees replaced so that she can exercise and loose weight. She has tried dieting and tablets. She is too scared to have a gastric byp so how is she ment to get this problem sorted?