Neighbor Killing Very Mature Willow Tree

My parents neighbor to the rear has a very mature willow tree that spreads over into at least three other yards (the tree is on the rear property line) Over the 20 + years that my parents lived in their home, we have cleaned lots of yard debris from the tree. Last year, the tree began dropping very large branches that had to be cut down with an electric saw. My parents did not complain to the owner, they just cleaned up the mess it made and repaired the arbor for their grape vine. This year, the large branches have began to fall again but in my next door neighbor’s yard. She spoke to the owner and he chopped up the branch for her. During the conversation with him, she learned that he has been intentionally killing the tree for a while. She shared this information with my father who is now very concerned about the potential damage the tree can inflict on our property.

I am sharing it with you all for advice because my dad seems to believe that a person cannot slowly kill of a tree. My parents home is four stories and the tree is taller than their house, so it is a huge tree.

I’m thinking that neighbor is going a cheap route for ridding himself of the tree. My dad wants to write a letter expressing his concern. But before he starts including inaccurate information, I decided to seek advice from other home owners.

Any insight on how to handle this is much appreciated. Thanks in advance