If I live where the neighbor wants a floodlight to illuminate their back yard - that light, by design, WILL somewhat light up mine - even if I would prefer it not to be (or don’t want the light coming in my window). I can get window shades and heavy curtains to help with the latter - but still -— my neighbor has a right to light his yard as well. It's a matter of design. Most people light their yards from their home outward. I had a neighbor like that in a previous neighborhood. He could have lit the property from the property lines and shone the light toward his house. I'm sure it would have been more expensive but it would have been much more "neighborly".
Similarly, I suspect most people have never even considered designing their outdoor lighting to be "sky friendly".
In the late 1980's, I was already selling HID fixtures that allowed the user to "cut" the emmitted light right at his property line. They were more expensive, but between the savings on electricity-use and labor (they didn't need to be replaced as often) they paid for themselves.
I just thought of something else: maybe the adjoining land-owner intends to light up the property past his property-line. In which case he's a prick.