Was he letting you know that he was licensed to GROW marijuana or use it as part of a medical treatment plan?  

So a wash.

And get this, when I confronted the neighbors about it, the husband bows his head and sleeks away; the wife tells me that since there was a big rain that washed debris from my yard to hers, and I was not there to clean it up, she didn't feel obligated to honor our agreement!  

These work really well, as long as they double them:https://www.amazon.com/Washing-Machine-Lint-Snare-Fabric/dp/B00COFAY78/ref=sr_1_13?

Then the cry was he couldn't afford any rent (I hadn't even mentioned an amount) because he was on disability. 

You would never expect your auto lender to come and wash the car for you.  

Most certainly not nearly as fast as it's going to wash by the tree and into your asset...and if you plant trees without real irrigation they're going to die.

And wash your hands...

I would think that potential extra income from in-unit wash/dryer along with ease of renting both units as stated previously would be best. 

I agree, it's $200 bucks it's a wash, get her out of there.