Bill C. writes:

The tree is in MA and I think that this is not legal. Any ideas on
how to best proceed to first save the tree and second stop this nut?

MA?

Call the authorities immediately. They will respond with truckloads of
hazmat removal crews in moon suits with hazmat barrels. They will dig out
all the dirt, containerize it, and put it in a toxic waste dump. Your
neighbor will get a six-figure bill. You will get a bitter enemy.

Or just forget about it. I'll bet the tree won't even notice. It takes a
lot of this stuff to have an effect, more than I expect was applied.