What to do when you get a job while on medicaid?

A friend of mine got accepted for Affinity from Medicaid a little more than 6 months ago. He was unemployed at that time living with his parents. Now, he landed a full time job, but they're paying him under the table. My question is: What should he tell the Doctor's office if he were to make an appointment? Should he tell them he is still unemployed or should he say he's is working now? Will working affect his medicaid qualifications? He complains about headaches and working in a office could be a cause of that, but he does not want to jeopardize his insurance. Thank you for your answers.