He can call for a police escort to go back and pick up his belongings.Change the locks.

Police reports, noise complaints, discussions re unwanted gatherings suggesting drug activity, complete list of tenants and phone numbers. 

Now personally I would take a "peace-maker" approach (the velvet-glove) and just contact the tenants and say how it's been a noticed things around there and not saying if it is them or not, but with it that noticeable out doors it's inviting serious problems and a person would be best to be smart and discrete of such a thing before that uninvited attention is police. 

If it isn't on paper, it didn't happen as far as police and courts are concerned.

The house got raided by police for drug use, and I ended up having to ask them to leave.

Cameras and police are ok ideas but remember he’s got all day to  damage stuff around your property. 

@Jessica West-Walker the police will get you a report, but that is about it.

We also have sign police, citizens having a part time job, paid a "bounty", you'll be paying the fine, the bounty if one, plus court costs. 

I worked in casinos dealing cards, while I don't have any type of security or police type background I grew some thick skin.

I encourage all others who have been swindled or have had similar experiences to share them too and to submit complaints to the Better Business Bureau, Department of Consumer Affairs, Consumer Financial Protection Bureau, State Board of Accountancy, Federal Trade Commission, Police Department’s Financial Crime Unit, and the District Attorney.