Thank you for your question about your friend, who, as I understand is suffering from chronic diarrhoea. The cause for most diarrhoeal illness is in the small or large bowels and the investigations should be concentrated in these areas. Most important cause in the tropics is due to infections and parasites and stool examination is vital. After excluding these, investigations lilke barium examinations of the small and large bowels as well as colonoscopy may be needed. Endoscopy of the stomach and duodenum has limited value except in diagnosis of coeliac disease where duodenal biopsy is 100% diagnostic. In your friend's case it is unlikely, as his blood for the relevent antibodies has been negative. But it should be recognised that the gold standard for diagnosing celiac disease is duodenal biopsy as very rarely the antibody levels may not be high. I suggest you seek an experts opinion locally.