The most common cause of diarrhea is a virus that infects the gut. The infection usually lasts for two days and is sometimes called "intestinal flu" or "stomach flu."
Since your father diarrhea is not related with any other symptom and the condition is good then it might be a part of IBS syndrome ,or an influence of the medication taken but if it lasts longer then he must check with a doctor to make further tests and know the cause to treat .
Diarrhea may also be caused by:
Infection by bacteria (the cause of most types of food poisoning)
Infections by other organisms
Eating foods that upset the digestive system
Allergies to certain foods
Diseases of the intestines (Crohn's disease, ulcerative colitis)
Malabsorption (where the body is unable to adequately absorb certain nutrients from the diet)
Digestive tract surgery
Diarrhea may also follow constipation, especially for people who have irritable bowel syndrome.