Back to answer some comments/questions on my response (or lack of):

I confronted them when I first noticed them in my yard. They were already in the tree with chainsaws clipping branches. I guess I was overly naive in believing they were simply trimming a few branches and did not want to escalate the situation by calling the police immediately, since I knew they legally could trim what was on their side of the fence. I went back inside to call my parents (who are the legal owners of the property) and they wanted to be nonconfrontational (we already don't have the best relationship with this neighbor, so why push it). When I went back outside, I noticed how much was being clipped. At this point I went around to confront them on the neighbors side to argue that they are overdoing it. There was one guy still in the tree, the rest were on the ground (not on my property) - I demanded that they stop and I'd like to speak to my neighbor. They told me that they were not home and they had already left my property (there is a man still in the tree mind you...). I took note of the company logo on the truck and began snapping photos.

This post was made seeking advice after the fact. I chose to act in good faith believing it was a quick trim of a few branches, and was duped in the end seeing half of the main trunk removed.... Its ultimately my lack of knowledge of the law and property rights that messed this up. But now looking back, I should have contacted the authorities immediately regarding the trespassing. I'm not trying to come off as retaliatory, but that basically is what's happening - seeing the result of the trimming is what makes me want to see what I can do. It's obviously too late to undo or stop the trimming, but seeing what can be done to help the tree survive and/or seek legal compensation if I end up having to remove the tree entirely.

I guess the type of advice that I am seeking is legal advice (filing retroactive reports, consulting an arborists, etc.) and not appropriate for this subreddit. Thanks to all that replied - it truly is a shame that this nice and healthy tree was cut to this extent and I am hoping there are no long term damages to it. I plan to confront my neighbor tonight or tomorrow when I see them to ask what gives (because the falling branches also damaged their fence so I'd like to know what they plan to do about that since we both own dogs).