One is an exclusion for Business Use of the property and also a specific exclusion for controlled substances.

If it is harmful you need to remediate. 

Okay if you want to file a lawsuit in America you have to prove adequate psychological, physical, or financial harm was done to you.

In a nutshell, I came here with a technical question which involves balancing his moving out and my finding a new tenant, hoping to find the answer to guide me so I don't screw him, and not harm my business at the same time.

----Many times, when you think you're helping people, you're really enabling an unsustainable lifestyle, and enabling them usually does more harm than good over the long run.

I do not ask tenants if they are smokers, I will rent to smokers, but it is very clear in my rental agreements and house rules that there is no smoking (of any substance) on the property (that includes garages, open space, parking areas) by tenants, their relatives, and their guests.  

No harm no foul.

@Art Maydan 1). from an insurance perspective, you could be held liable if a tenant is harmed because of non working life safety equipment.  

I had some of my buddies go to rehab and such for substance.

@Sonia Spangenberg If the city is giving you issues, you could ask them if you gave them an environmental report showing that the land is safe and added septics is doable and won't harm the water table that might get them to calm down.