I arrived at the crag and saw a team on Great Western, the second was unable to follow the route. Shortly after this I had a large crash and screams, the lead climber had abseiled to remove his protection the second could not recover do to failing to follow. The casualty feel roughly 30 feet onto a large boulder and then a further 7 feet on to the ground, he was wearing a helmet which clearly saved him from serious head injuries. The casualty appeared to have a broken arm, wrist, leg, back and possible internal injuries. Working with some other visitors to the crag we provided first aid and I contact the air ambulance for emergency treatment and evacuation. Discussion the situation with his second it appears a back up piece of gear for his abseil was removed leaving a single cam in a pocket, which swing into to get the gear this failed.