My athletic friend's lung collapsed?

Possible. Sometimes this happens in young, tall and athletic individuals for no apparent reason. Sometimes associated with an underlying lung disease, but generally it is a mechanical problem. Sometimes it is recurrent.
Best wishes. Our best wishes to you and your friend. The risk factors for primary and secondary spontaneous pneumothorax (sp) include: smoking (including marijuana), tall thin stature in otherwise healthy, connective tissue disorders, pregnancy, familial history. Lung inflammation is also hypothesized to be important.