Forgot to add pet policy on lease agreement

[Sierra Fox]

Hi everyone, I discovered a major rookie mistake on my lease agreement with my current tenant who signed July 26th.  I completely left out the pet policy in the lease agreement after making adjustments to the new lease.  I advertised the home as "strictly no pets" and she didn't have any at the time.  I came across a facebook post that she publicly posted regarding a male puppy appears to be a pit bull or a mix which she is supposed to be getting on September 1st.   

How do I handle this?  Am I able to legally add a pet policy since I advertised "no pets"?  She never contacted me once regarding getting this puppy at all.  I just happened to see she popped on my friends you may know - and it was her.