I know in Virginia my friend had a similar issue (side-by-side duplex, though, not second floor) and he was not proactive, just warning them several times verbally.

I'd also suggest speaking about Real Estate with friends, family, neighbors, and anyone else in your sphere of influence. 

I also have a friend/sub-contractor with 40 years experience in all areas of construction who works with me on most of my deals for low cost labor.

Wonderful reflection my friend! 

It's usually a very bad idea to go into business with friends or family. 

Do you know the area very well or have family/friends that live in the area?

The energy in person is different for me, and I've made some of my best connections and friends at Chicago meetups.Here is a list of the Chicago event from BP: https://www.biggerpockets.com/forums/521-real-estate-events-...I also have a list of ALL THE CHICAGO MEETUPS that I am happy to share with you. 

I looks like you are in Cali which is a notoriously tenant friendly state.

Next, analyze deals with an investor-friendly agent. 

Best part is NJ is not what its made out to be in terms of landlord friendly, the state gets portrayed as the entire state being bad when really most wacky laws are in the major cities and they apply to 5 units or more.  2-4 unit multi families fly below radar per say.