stressless, Are these in a region of faulting? Was there an earthquake around the time of these events - even a small 2 to 3 magnitude quake that some might not notice but would be picked up on seismographs?
Not that I'm aware - as I said above, the neighbors pond was just flowing over the dam/road and finally cut thru to create a catastrophic washout of his dam, which allowed the 'crack' to gain water velocity which washed and enlarged the crack.
*Assumption - fixing the dam on his side, NP + raising NP 1' will provide hydro relief to the flow from LP
*Assumption - filling the crack with bentonite/mud after sealing his end, NP, to the mouth of the crack on LP will seal the crack
*Assumption - putting a bentonite cap at the entrance of the crack's plug on LP and covering with 1' of clay top to bottom and 30' on either side should prevent another seep
*Assumption - putting overflows on both ponds, neither had one prior, will prevent overfilling and possible rupture above the sealed area