If they ask for a reasonable accommodation based on a disability, they have to meet the definition of disabled and the request must be reasonable.  

In response to a much too lenient (in my opinion) proposal to accommodate her situation, she gets bent out of shape.  

@Paul Steward show me where ADA requires someone to undergo significant expense to accommodate?

If you intend to maintain a no-pet policy as an accommodation for someone with severe pet allergies, it would be smart to have them provide a note from their doctor verifying their need for a pet-free environment.

I wanted to accommodate with the fence, but I see how we may have opened Pandora's box, and need to shut it.

But what I am more curious about is what pain points you have experienced in whatever market it is that you invest.What things can an agent or property manager do to better accommodate your needs?

And our expert answer is, we don't know.So try to treat this situation as if it were your grandma living in your rental (assuming you like your grandma) and try to be as accommodating, and reasonable as possible. 

I know that my rentals CAN accommodate more people if someone sleeps on the sofa(s), the floor, in the basement, and in a large closet someone could use as a bedroom, but I reserve my rights to ask who will be living in the home.

One thing that people forget is that someone with a service or ES animal is asking for an accommodation to have the animal at the house. 

No you can not paint the ceiling black to accommodate you inability to sleep at night.