Final answer:
The nurse instructs the client to follow a low-fat diet for 72 hours before the fecal fat test.
Step-by-step explanation:
The nurse instructs the client to follow a low-fat diet for 72 hours before the fecal fat test. This is because the client experiences pain when eating fats and the test is used to assess the normal function of the gastro-intestinal tract and fat absorption. By following a low-fat diet, the client can minimize the symptoms and provide accurate test results.