The lungs are contained within the pleural membrane in the chest cavity. The pleura lines the exterior of the lungs and the interior of the chest cavity with a potential space between the two layers. If air enters the area between the pleural tissues, the potential space becomes an actual space. If the space expands, the lung expansion is reduced, and respiration is compromised.
If the air comes from an interior wound, it is called a closed pneumothorax, and if the wound is external, it is called an open pneumothorax. The opening may be the result of a lung defect or an injury (trauma). The larger the hole, the more rapid the progression of the pneumothorax. If the hole is small, the pneumothorax may self- seal and the body will gradually reabsorb the air. If the interior wound does not close and air continues to enter the space, the condition is called tension pneumothorax. This condition is very serious and requires emergency treatment.
You tell us that this young man sustained a fractured rib and a pneumothorax from a sports injury. We assume a blunt trauma caused the rib fracture and the pneumothorax, but that the broken rib did not penetrate the pleura and lung. A pneumothorax can result from blunt trauma to the chest, causing the pleura to tear, much like popping an inflated paper bag. What is the chance of this area rupturing again, producing a spontaneous pneumothorax? If the mechanism of the injury was as I have described it, the injured area should have healed adequately with very little risk of a spontaneous pneumothorax from this injury. The lung itself probably suffered little if any damage from this injury.
However, if a chest injury results in penetration of the lung by a broken rib, a gunshot, a knife or a similar mechanism, then the underlying lung tissue is damaged, and scarring of the lung and pleura may increase the risk of pulmonary barotrauma with diving. A person with this type injury would require an evaluation prior to scuba diving and probably would be advised that the risk of barotrauma is too great to consider diving.
A spontaneous pneumothorax can occur in an apparently healthy person with no warning. This is due to a defect in the lung which may be congenital or may have appeared later in life. The recurrence rate of this type pneumothorax is high, and for that reason these individuals are usually advised not to dive. The risk is that a spontaneous pneumothorax might occur while diving, resulting in a closed air space which could not be equalized as the diver ascends. The resulting expansion of this air space with decreasing ambient pressure would interfere with the function of the heart and the other lung, with possible disastrous consequences.