Nasty kids next door?

Whilst out in the garden the other morning, i noticed that the two nasty kids next door were smearing fresh Dog s*** over the rungs of there window cleaners ladder "with the aid of a stick" whilst he was cleaning the bedroom windows. Obviously the window cleaner was unaware of this trick on his descent down the ladder. Question is, should i have warned him before he descended the ladder or was it ok for me to ignore this ghastly happening, not wanting to be a nosey neighbour and all?