First, I would suggest she continue to see a doctor so that the cause may be found and treated.
As to a diet, until active symptoms subside either a low or high fiber nutrition therapy would help. Which nutrition therapy she should use depends on her symptoms. If constipation is the problem, eat high fiber foods. If diarrhea is the problem, eliminate high fiber foods, milk and lactose-containing dairy foods (See milk topic). High fiber foods are raw vegetables and fruit, whole wheat bread and cereal grains. These foods would stimulate the intestinal peristalsis (rhythmic movement). Caffeine may also contribute to symptoms and should be limited for a short term.