Dear Ms. diamond,
I would like to know:
What other symptoms she has aside from diarrhea?
Was there no diarrhea before the surgery?
Did she start diarrhea right after the surgery?
How her stools look like?
What investigations have you done?
Diarrhea is not a common problem after hiatal hernia surgery. However, in general, any surgery in the abdomen or gastrointestinal tract is known to cause a chronic diarrhea.
She needs various investigations to arrive to a diagnosis. She needs to go back to gastroenterologist, and do stools analysis and culture including electrolytes in stools. She also has to run blood tests, and an endoscopic examinations may be required. Be careful with the foods she eats and some medicine can exacerbate her diarrhea.
Do not expect though that extensive investigations will give an answer to what is the cause of her constipation. Chronic diarrhea can occur while the cause remains unknown.