A local Spanish climber who was not part of our group was on route and approximately at the height of the third bolt. There was a clear view of all of the route from the belay. Also the belayer was wearing a bicycle helmet and the rope and gri gri they were using didn't appear worn. This would suggest that they were new to sport climbing. Whilst they were on route a member of our group came over and commented on the belayer's use of their gri gri. It appeared they were only using one hand, pulling slack through and then letting it hang, not keeping a hand on the brake rope. Almost immediately after this comment was made, the climber fell. Their belayer panicked and grabbed the live rope which ran through and burned their hand. The climber tumbled and span down the wall until the belayer grabbed the brake rope and stopped them around 2m from the floor. After the climber gathered themself, they got back on the route and climbed to their furthest quickdraw. From here they downclimbed whilst unclipping, shaking all the while and nearly falling off again. The belayer continued to use the gri gri one-handed as one of them was now injured. Shortly after they both left having not done any more climbing.