Can a 16-year-old with severe untreated marfan's die due to too much exercise?

Not exactly. Marfan's patients need to choose their form of exercise in consultation with a specialist. The aorta is prone to tear, and this is a consideration in choosing what the person will do. No one really wants to go through life without the enjoyment of physical exercise, and a marfan person need not forgo something this important.
Yes. Marfan's syndrome is a connective tissue disorder which weakens the wall of the aorta. This most commonly leads to a tear in the aorta or an aortic dissection. This can also lead to an aortic aneurysm. If you know you have marfan's you should be evaluated with a ct angiogram by a cardiac surgeon to determine if exercise is safe.