My Sister Is 68 Years Of Age, She Poops All

My sister is 68 years of age, she poops all the time, at least 4 to 5 times a day and large movements. Her family Dr. told her it`s impossible to poop more than you eat, yet she is gradually going away to nothing. She is approximately 5ft 5 and now weighs 90 pds. Something is very wrong with her and she looks like a walking skeleton. What do you think is going on with her, and what kind of specialist should she seek?