YES weak heart does cause back pressure to build up and cusing the legs+ feet to swell up
Check her kidney
function test , because weak heart leads to weak kidneys and they both increase swelling.
Increasing furosemide+ spirinolactone + digoxin
to improve cardiac contratility and hence less leg swelling.
Due to chronic swelling of legs and feet , people complaint of itching and spasm during nigh , a pneumatic leg compression machine or ITALIAN tigh leg stocking at day time will help decrease swelling.
Hope this helps.