The potential for contamination via runoff from pools and spas is known so a call to the local reguatory agency might be a way to get the neighbor to take notice. If not, a claim for the damage to the neighbor's insurance carrier or litigation for the damage could be a possible action.
With friends like Guido, you will not have enemies for long.
“Intellect is invisible to the man who has none”
"The difference between genius and stupidity is that genius has its limits."