What Causes Diarrhoea, Lack Of Appetite And Tiredness?

I have a friend that can not eat well (normally eats really well) and has diarrhea and is tired all the time. We thought he had food poisoning but it has been over seven days now and nothing has changed. He is really tied, irritable, he can not sleep well (when I mean well I mean basically not at all) hence the irritability.