I am not a gardener but wanted some advise from this forum

I think my neighbour is trying to kill a tree that is just on my side of the
fence. I would estimate it is 10-15 years old and each year it blocks out
more of the light from his patio. It all abit of along story but I am
convinced he is doing something. The tree is still healthy but I want to
know what to look out for. What are the choice of methods and what will be
the subsequent effect. He is always around the tree at times when he thinks
I am out or at night. I have inspected but cannot see anything. There is
awhole which looks man made at the base which might have been filled. what
could he have put in there.

As you will appreciate I am most anoyed....can anyone help please

Ian