I am CWI with abseil modeule. I was on an in house refresher, sogning up to volunteer at Giys Farm as a climbing and abseil instructir. The assessor offered to demonstrate the abseil setup to me used at Guys. She set up a releasable abseil correctly but then prepared to abseil down the tower herself with no safety line attached. As she was backing over the edge through the open gate, I asked her to double check the setup. It was only then that she noticed she was backing onto the three meter tail end of the rope that had no stopper knot. Had she continued, she would have fallen directly off the end of the rope to the ground. I prevented a serious accident.