Quote from @Nathan Gesner:
Does your lease state - in writing - that pets are not allowed? If so, this is a violation and she should be fined or you demand the dog be removed.
However, she probably knows how to play the game and can get a doctor to write a letter designating the animal as an ESA. If she provides the evidence, you'll have to allow it.
Nathan, how would you determine an appropriate fine ($200, $500, $1000..?) and/or subsequent pet deposit and fee, if the Lease stated No pets but they then got one? I have wondered about this for units where it clearly does not allow, and so no additional pet deposit or fee is included. A post incident pet Addendum with this information for both parties to sign and agree to ...or if not agreed to the final alternative being breach of Lease and ability to give notice if it's really that worrisome to the Landlord?
Or in the future, include the pet addendum at lease signing even if policy is no pets? Doing this at lease signing just seems to give a message that at one point you'd be willing to consider and that it's an option.