My cousin has a special needs twelve year old, and let me tell you he has the strength of a large buff man! He nearly took my trailered 23 ft boat off its chocks (whatever you call it when you put the front of the boat trailer on the donught on the ground).
I would venture to say that it is never appropriate to bring a special needs boy in a ladies' shower when there are ladies in there. Like others have mentioned, assuming this boy is special needs, in the very least the mother could have 1) waited until she knew the showers were empty 2) asked CG manager for blocked off time 3)showered him using the trailer shower 4) used the hose attachment and showered him outside of trailer with his swim shorts on.