My husband broke a rib from a light cough about six months ago and the same thing happened yesterday. What would cause thisi he smokes

Depends. A cough may cause rib injury or injury to the muscles around and between the ribs. This may be mild injury but can also be significant including a rib fracture. A cough may also be related to a lung problem which may or may not be associated with pleuritis which is inflammation of the pleura which is a thin lining around the lungs & this can be painful. I recommend appropriate medical evaluation.