Inexperienced adult climber (in our party) got to top of a easy sport climb on their first outdoor ascent (on top rope). Once at the top, while getting ready to lower off they reached above the lower-off and grabbed onto a ledge to steady themselves, this dislodged a block of rock, sending it 12m+ down the crag. I was buddy checking a climber, ready to lead another route and we were stood at the base of the crag (2 routes away from the top rope, so prob 4m to the side). The rock, about 200mm in diameter landed between us, striking the back of the climbers leg & ankle a glancing blow and smashing on the floor between us. We were very shaken by this and the climber was in immediate pain. The rock landed in the 1/2m space between us (both wearing helmets) which felt very lucky as a blow to the head would have been serious.