An adult, non parent, had set up a bottom rope for a teen, the teen top roped it twice. then went to clean the anchor. at the anchor the teen connected a quickdraw on to the lower off untied the rope and tied it to another draw on his harness. I was on a near by route with a clear view, seeingthe confusion on the teenagers face I waited on a hold for a few moments. his belayer had LOADS of slack out and no hands on the rope, just leaning against a tree with arms folded. shouting up something unclear. The teen only connected by a a quickdraw to his harness started trying to unclip this only point... this is when I shouted over what to do, in order to prevent things getting worse the climber returned to the ground safety went and left shortly after.