Your powers of observation are legitimate enough, in my opinion.If your friend saw mail in a way that wasn't illegal, he could be a witness.  

@Dean Weltman I observed the same thing, those from CA were bashing the NY Post and one suggested the NY Times would be a better source.

I didn't monitor any of my tenants until/if I got a complaint from another tenant or I just happened to observe or hear some behavior that I needed to give them a warning about.So, in my case, since I lived in the same building, if they were throwing a loud party, I'd usually see or hear it.  

Now I have to pay a Marshal to observe them.

As a disclaimer though, that is just an observation, you should never just run a credit check on your applicants.

Just an observation - it would require a lot of management.  

I doubt the police would do anything based on your observation and could be a problem for you if you produce those pictures.  

MA, TN, MD, NJ all have different rules, but we all observe "landlord/tenant law" per se.Google "Nolo".

I let tenants list any observations like that, then I ask them what specifically if they something fixed.

These are things that i will continue to study and observe as i move into this type of REI.