My friend has been coughing for about 3 to 4 weeks now. the cough is a dry cough and it comes and goes. what could this be?

Coughing: there are dozens of causes of dry cough. In coughing the tine oars or cilia that normally move the bacteria out of the lungs are lazy or slow; so then you cough. Try lots of liquids, esp lemon/lime/honey/green tea. Steam inhalliation with tongue out is good Drink till urine turns light. Humming vibration shakes up the thick phlegm. See www.grossaninstitute.com