Then, they can call the police and get her for harassment.  

And now you are trying to police the tenants' use of who is using the washers/dryers.And if they're in-unit, you are now paying for the water for their relatives to come over and use their washers.And if you try to charge them an additional fee if they put in their own washers, they will try to hide them. 

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.  

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

The folks detained him before he can flee till the police came to put him in custody.

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

The gates will be better than climbing fences.My 2nd step would be to post "Private Property - Do not Trespass"-type signs and call the police if the neighbors do not respect the property.  

When a tk provider would present us with a property we would call the police department to ask them about the neighborhood the property was located in because it wasn't in our interest to be in a war zone or bad area.

Call the police? 

After they left the wife called the police.