Ahhhhh . . the friend-of-a-tenant question.
Your place does not allow pets. Your tenant is not disabled. But they have a visitor who has a disability and would like to bring a pet onto the property. What can you do (or can you not do)?
As a general rule, remember that rules of "no pets" must be amended or accommodated for persons with a disability making a request for a reasonable accommodation to use and enjoy the premises. For more information on the rules and exceptions here is a good review from the federal office of Housing and Urban Development (HUD) on service animals.
Clearly if a disabled tenant is making a reasonable accommodation request to have an assistance animal in a "no pets" property you would have to amend your rules, polices or practices to allow that.
But now, back to a friend or visitor:
The federal Fair Housing Act states that the law applies to a person residing at the property "or any person associated with that person."
See excerpt of federal law below:
"42 U.S. Code § 3604 (f) (2) To discriminate against any person in the terms, conditions, or privileges of sale or rental of a dwelling, or in the provision of services or facilities in connection with such dwelling, because of a handicap of—
(A) that person; or
(B) a person residing in or intending to reside in that dwelling after it is so sold, rented, or made available; or
(C) any person associated with that person."
Since the law specifically covers persons in addition to tenants, I would certainly provide the same accommodations and allow a visitor the same benefits that a tenant has.
In short, so long as their request is reasonable, a visitor would likely be able to bring in and visit a "no pets" building with their assistance animal.