Marfan syndrome is a weakness on the connective tissues, this means that numerous if not all tissues of the body are weak and is very easily to get injured, but I wonder, does Marfan syndrome prevents the grow of muscle?
It certainly does not completely prevent it. One of my roommates had Marfan's and, while lean, he had enough strength to be able to hurl a 200-lb male across the room (he also had a bit of a temper on him). He did not have a workout regimen, but had grown up on a farm, so he probably grew up with a fair amount of physical activity.
I suspect that someone with Marfans would have to be careful about over-stressing their tendons, but could still develop muscle.