Can people with marfan syndrome still fly on planes?

Usually yes: A Marfan syndrome person with a current or recent pneumothorax (collapsed lung) should not fly (unless the airplane stays close to his airport's altitude) because the decrease in outside air pressure as the airplane ascends can allow his pneumothorax to enlarge and compress the lungs and heart.

Yes: The only abnormality that might affect the ability to fly in patients with Marfan syndrome would be if there are very large, unstable blebs in the lungs. This is an uncommon part of Marfan syndrome. Even in patients with pulmonary blebs and Marfan syndrome, most typically, there would be no problems with flight.