She has called the police to have him removed but they said they can't do anything it's a civil matter.

I also think that in places like CA there are larger companies with enough $ and willing to use it that invest highly in rental real estate that will tie up extreme laws in litigation for decades, well past the virus situation.

If you default or have to be in litigation with the seller you are misrepresenting your self and information contained in the application it can result in civil liability and fraud. 

I'm a PE (Civil, but not structural), and you wouldn't want me to stamp an electrical design

Take a look at the governors order https://www.governor.nh.gov/news-media/orders-2020/documents/2020-08.pdfYou may be liable for civil penalties if you try and force the tenant out whether "at will" or annual lease tenant. 

Are there any ongoing litigations, and how litigations have been handled in the pass.

., ...Death, disability, divorce or separation, job relocation, financial problems ----> "pre or post" bankruptcy, legal problems ----> civil suits, criminal actions, ...and more!)

. - He will build the homes, including working with the architect and civil engineer to value engineer some unique concept homes with a factory.

@Tamar Hermes I don’t for a reason that most people don’t talk about, litigation.

Before I "try" to have a civil discussion with the neighbor (this is a D+ neighborhood so I don't know how civil this guy will be), I wanted to get some recommendations.