Pennsylvania Boundary Treeline Dispute

Hello all,

I recently purchased a home in Pennsylvania. In the backyard, directly on the property line shared with my neighbor, are a row of pine trees. They are currently about 40-50 feet high. The pine trees are about 20 years old, and based on conversations I've had with two different arborists, they are healthy. However, these trees were planted quite close together (maybe 5 feet or so in between each tree).

Around the same time as these trees were planted, this neighbor installed a pool and a shed in their backyard. Both the pool and the shed sit in the shadow of the row of pine trees. My neighbor approached me a few different times concerning these pine trees.

He has two issues:

1) The wind blows significant amounts of pine needles etc into their yard and into their pool. These needles also clog rain gutters around the shed.

2) He has concerns about the trees falling on their pool/shed. This has happened in the past when one of the healthy trees fell due to extremely high windows.

I don't have any concerns with the tree line. However, in the interest of being a good neighbor, and to help mitigate risk to their pool/shed, I am looking at two options:

Option #1 is to remove the entire treeline and replace it with a new privacy hedge.

Option #2 is to reduce the height of the trees by 10-15%. While I was at first concerned this would stress/kill the treeline, the arborist that recommended this said it would not cause excessive stress as long as the cuts were limited to the areas near the top.

The neighbor wants to go with Option #1. However, they also want me to bear the significant majority of the cost of this option. They also said that if I go with Option #2, they would remove all the branches on their side of the property line - all the way to the trunk.

The question: I know tree trimming is permitted to the extent the branches are over their property. However, what they are describing/threatening would maim, destabilize, and likely kill the trees. Are they permitted to do this? Any speculation on how insurance companies would view this if the destabilized trees fell on their pool in the future?

Thank you

Location: Pennsylvania