Helios was not a rapid depresurization. We had the same thing happen in a Falcon 20 and I noticed at about 7,000 ft our cabin was climbing with the aircraft rate of climb so advised the captain. He didn't believe me so said the cabin altitude high light will come on at 10,000 ft. which it did. Since he kept climbing I said the masks will drop at 14,000 ft so finally he leveled off. After some troubleshooting with no sucess we returned for landing. The outflow valve was stuck open.