Pruning - even really bad pruning - will rarely kill anything and I doubt the removal of a few branches would qualify :-) There may well be something else wrong that causing issues....can you post photos?

Although a bit presumptuous by venturing onto your property to do so, your neighbor actually did you and the tree a favor by cutting the branches off at the trunk. That is far better for the tree than leaving a stub with a partially cut branch.

Your local laws would dictate any remedy. You likely would need before and after pix and be able to prove the trimming killed it. Any pix of the aftermath?

tj

It's far from "a bit presumptuous," in most places, it's illegal. And it's certainly absolutely disgusting for a neighbor to invade your property and cut back your trees.

Research tree law for what you can do and contact a lawyer. They may owe you up to and including the price of a replacement tree. Which could be exceptionally hefty for a 50-year-old pine. This is a particularly fun area of law as neighbors may not trespass to cut back any tree you own. In some places, they can't even cut back a tree without your permission. It just depends.

Yes, you'll replace it with a sapling. That's not the point; they killed a fifty-year-old pine. They owe you for the tree.

You would require proof but, if they've admitted this to you and others, that would do. And get a tree surgeon in to inspect the tree. Improper cutting can certainly introduce disease through the cuts, regardless of where the cut was made, damaging or killing the tree.

Whether it can be saved or not is another question that an expert would be able to answer. By the time it appears to be dying to the casual eye...well...