There is a unanimity of medical opinion that smoking marijuana is absolutely contraindicated in asthma. There may be other psychological reasons why your friend persists in this irrational behavior. I would strongly recommend that your friend have an open discussion of this issue with his doctor. Referral to a psychiatrist that is skilled in handling problems in patients with chronic illness such as asthma may be warranted. This is a complex issue.
Some individuals have asthma which only accompanies exercise.
Inhaling cold dry air can provoke bronchospasm in a large percentage of asthmatics. This response can be blocked by treating with 2 puffs of albuterol or cromoyln 15 minutes before exercise. On the positive side regular aerobic exercise exerts beneficial effects on the lungs of asthmatics just as it does in normal lungs. Asthmatics can increase their pulmonary function as well as pulmonary reserve with regular aerobic exercise such as running, cycling, and swimming. Swimming is especially good as it doesn't have much tendency to exacerbate asthma.