Didn't include no-pet clause In the lease - could be problematic

[Marshall Rousseau]

This has not yet developed into an issue, but I feel like I may have screwed myself over. The tenant knows that she is not allowed own any pets on the property because it was made clear in person and when the unit was being advertised.  However; I did fail to include it in the lease. although, I don't think she is aware that its missing from the lease. I discovered that it was missing after she texted me to ask if her friend (who owns a dog) can come stay the night (dog included) one to two times a month. I was going to reply that, per the lease, I do not want a dog staying overnight on a reoccurring basis. (I would be lenient if it was a one time thing). I was about to text her back when I decided that I should check the lease first.... and low-and-behold I did not include anything that specifies pets are not allowed. I use aprtments.com and they do not currently allow lease templates, so I have to make one from scratch every time and this time it was missed. 

How should I proceed? Can I still tell her no and hope that she doesn't realize that legally she can do whatever she wants? - do I tell her yes, so that she doesn't feel the need to look into the details on the lease .  Am I honest with her an say its not specified on the lease, but it should be and that i am (or am not) ok with her having her friends dog over, but I want her to sign an amendment saying no pets are allowed?  Not sure what the best path forward would be and I would appreciate any advice!