My Mother Is 81 And Has Been Suffering From Months

My mother is 81 and has been suffering from months long bouts of severe, uncontrollable diarrhea for over a year. There is NO abdominal pain or nausea, only the diarrhea. She has lost 30lbs on a 4'10", and is very weak. She has been to her PCP and a gastroenterologist, has had an abdominal scan(the prep kicked off another bout) and has had multiple tests. She was scheduled for a colonoscopy, but was afraid the prep would literally do her in. They can't tell her what's wrong, and it seems they don't realize how sick she is. Enzymes and anti-diarrhea meds help, but only for a short period of time. Any advice would be so greatly appreciated. Thank you.