What you are dealing with is a balance between fluid overload and dehydration which is a common problem in the elderly who have heart failure. The heart failure results in the heart being unable to pump blood and so fluid backs up into swollen legs and eventually lungs. For this reason, the heart doctor will recommend fluid restricting. The side effect of this is dehydration and constipation which can actually have very detrimental effects. The best advise would be to seek out the guidance of a geriatrician who specializes in care for the elderly and can help to discuss together with your mother, her heart doctor and everyone what the overall goals of care are in order to help her be most in line with health as well as comfort. You are right to pose the question - the answer needs to be flushed out in a multidisciplinary approach.
Please let me know if you have more questions.