What Causes Persistent Non Productive Cough?

Hi, my friend's mum that lives in Nigeria has had this cough for over four years now. She has been to several hospitals and results have shown that nothing is wrong with her. She has used different prescribed drugs but they don't seem to work. The cough is dry and usually comes in the morning and at night. She does not cough out blood and her health does not seem to be getting worse or better....please what can she do or what can you prescribe.