Diarrhea can be due to many things such as medications, trouble absorbing food ("malabsorption"), infection, or structural changes in the bowel. You are absolutely right to wonder if augmentin caused your Mum's diarrhea. Augmentin is an antibiotic that commonly will reduce the normal bacteria that lives in the bowel. (There is normal bacteria that functions to help digest food and bacteria that can cause infections in the bowel.) Diarrhea is a common side effect.
Augmentin and other antibiotics can also be responsible for changing the environment of the intestine and allowing a pathogenic ("bad') bacteria to grow up. The most common problem bacteria is called Clostridium Difficile (or "C. Diff" for short.) C. Diff releases a toxin that can cause irritation and damage to the bowel. The symptoms can be cramping and smelly, watery diarrhea. Untreated, people can get very ill and develop fevers, dehydration, and rarely swelling of the bowel (called "toxic megacolon" which is an emergency.)It is very easy to make a diagnosis. A stool sample is sent and tested for the C.Diff toxin. The conditon is treated with either metronidazole or clindamycin - both are oral antibiotics that kill C. Diff.
Finally, it is possible that the bowel bypass is the cause of your Mum's diarrhea. This occurs because the food has less bowel in which to be digested. Until the shorter bowel learns how to be more efficient at digestion, people will experience diarrhea. In this case, the treatment is supportive. It is important to make sure your Mum is drinking enough fluids so that she does not get dehydrated. Sometimes adding an agent to slow down bowel transit time (such as tincture of opium) is helpful . I would not recommend that initially because of your mother's history of bowel obstruction.
Welcome Doc! and we, on the Ovarian Suppport site appreciate your detailed answer! Looks like they are lining up for your info! MV