Entitled next-door neighbour constantly pushes neighbours’ generosity by using his lawn and properties, when confronted about it: ‘He laughed it off, said “we’re neighbours, what’s mine is yours” smiled and kept sitting there’’ – FAIL Blog

The ‘neighbourly’ argument is just gaslighting; it’s obvious that being neighbourly is about sharing and peacefully cohabitating, and not about letting someone come into your house without permission and using your stuff.

There is a short story by Julio Cortázar that describes a situation just like this: two brothers live in a big old house, and the house starts being invaded by an undefined presence, at first they let it, leaving rooms behind for the presence to live in, and limiting themselves to one or two rooms for them, until the thing occupies so much space they have to completely abandon their house.

Of course, this is a fictional story and has magical and ridiculous elements to it, but it works wonders as a cautionary tale about things like what happened to our friend.