When you first sent that on FB Steve I thought you meant the tile roof, that he wanted that cleaned.
If its just the glazed roof then in reality it could be done with a triple extending ladder with stand-off fitted to keep you away from the roof tiles and gain extra work-space. It would be fine on the lawn IMO, just explain to whatshisface that there’ll be a few divets in his grass. The two sides adjacent to the roof valley could be done standing in the valley, although you might need wellies if the water doesn’t run particularly effectively and you’re left standing in 3 inches of leaf grime.
Apart from that, it’s a cherry picker but you’ll bugger his lawn with a tracked one however, it’d be a lot more effective and enable far better access than working off a ladder.