Hi there andrewnelson: Am I understanding this correctly........you rented the home based on the fact(s) that the it was pet friendly (meaning pets were acceptable) only to find out later on they was not the case? Where exactly did it say it was pet friendly? Was it in the written lease, verbal okay, or otherwise implied? Were there any witnesses? If you have already signed a lease based on the assumption from the landlord saying it was allowed, it is the LANDLORD that has some explaining to do. I would also consult an attorney BEFORE confronting the landlord, Many attorney's charge nothing for an initial consultation. Know your rights, legally before approaching this landlord, or else he/she will try to ram-rod you thinking you don't know your rights, as many people do not. Also, document everything and anything that is said during all conversations between the landlord and yourself. I guess you can tell I am very touchy about this subject, as I have been though this same thing in my younger college days, and I was taken for a ride. Literally! I wish you the best, and STAND YOUR GROUND.
The best thing to do would be to talk to the tenant about the mix-up. The two of you should then agree on what the options are for both of you. You could talk to a lawyer for legal advice on the issue.
We own a condo where we owners can have a pet. But under the by-laws of the Association, it states No pets by renters. If an owner saw that my renter had a pet, that owner could complain to the Board. But that's about all that can be done. So read your Association Rules. I'm sure it explains what you have asked us.
If it is a small pet, you could try going to the Board and asking for an exception. Prepare you case with references from your tenant's former landlords. At best, you will have to allow the tenant to move out without penalty if that is his or her desire. At worst, you will have to pay them to move out. What is written is law unless you can get an exception.