As your tenant is older the argument can be made that she needs this modification for health reasons so you likely don't really have a choice.

IRS Offers hardship exclusions for things like relocating at work, health reasons, divorce etc.

My goals are at least $500 a month so that I can retire overseas (where life is cheaper) due to my serious health issues that prevent me from working. 

AntifragileScaling Up (this book had a big impact on my non-real estate business)Americana: a four hundred year history of American capitalismE Myth Revisited (although I would skip or skim the first third of the book)The Knack: how street smart entrpreneurs learn to handle whatever comes upOther random selections:HealthThe Good Gut (interesting look at our gut microbiota and how it impacts our overall health by two Stanford immunologists)Becoming A Supple Leopard or Deskbound (both by Starret)InspirationalThe BibleUnbroken

The majority of the tenants in our properties, 80%, are in health care and have been so for longer than a decade.

She has owned it 50+ years and is now going to move into a care facility because of her health issues.

If surfaces start turning black that aren’t supposed to be you would like to know.To answer their question above I would admit it is an uncommon provision in this area but you like to include it as it protects the health of the tenants and protects the property.  

And of course, if there was a job loss or health issue involved, compassion should be considered.

Those reasons approach infinity deriving from mortgage servicing contract restrictions, portfolio size, balance sheet health of the investor and regulations to name a few.

I have my realtor's license and currently work as a PM in Commercial Property Management managing a large health care portfolio.