Unapproved Pet

I once had a renter sneak a pet into one of my rental units too. My situation was a little different as it ended in an eviction. My tenant refused to get rid of her dog, so I got rid of her. In my case she lost her entire deposit due to breach of agreement. However, your tenant is planning on moving, so that poses another issue.
So, how do you charge the deposit for a unauthorized pet? First, keep in mind the California laws governing deposits. You must provide a breakdown of charges within 21 days. So, charge what you feel is justified, but remember the tenant can request receipts or proof of charges. In this case, I would look for any damages caused by the pet such as: Pet urine, Dander, Fur, Chewed Carpet, Etc. Then charge your tenant for each item accordingly.
If the pet deposit is spelled out in your lease, I would suggest charging her the fee. ($250) Not only can you charge for the cost of use of the steam cleaner but you can also charge a fair hourly rate for your time, plus chemical expenses etc.