It doesn’t matter if there is zero intent to discriminate in carrying out such polices.

It's a given someone will block someone, and you'll be dealing with policing the parking situation on a regular basis, says my crystal ball.If you can afford to create more off-street parking, you'll have less tenant issues, and the units will be easier to rent.

Should he encourage the existing tenant to call the police/gather more/better evidence?

I would file a police report.

Also, I would possibly contact the police and see what their view is.

Call the Police!!

If the duplex is:1 - in a really  bad area 2 - not along the edge of the bad area3 - is not in an area designated as a redevelopment zone4 - in a neighborhood that has police trust issues associated with it.Then I wouldn't touch it.

Both work for HOA to act as police force.

When you put it like that, people will either not want to get involved, or they will become major dramaholics.If a crime has been committed, tell the police and let them do the investigating.I've been screwed by a management company, as has my daughter - before my days of being a manager.  

the police officer said it is the law to have your pets leashed and maintained.