The whole thing still makes little sense to me. Most people plant
privacy screens that are close to the property line and then just mow
up to their side of it, not both sides, with a wasted buffer strip on
the far side. It's also less work. The trees, while planted on one
sides property, often have branches that extend a reasonable distance
into the other party's property. This only seems fair, as both
parties are benefitting from the privacy screen, so to have it take up
some space from both sides is not unreasonable.

If I had your lot size with plenty of space, I'd be very happy with
the privacy screen your neighbor has provided. Legally, you can trim
it to your line, but I fail to see why anyone would want to do it.
BTW, how big is your neighbors yard compared to yours?