You will need to consult with an attorney, because the facts need to be clarified. An attorney will need to look at the paperwork drawn up by her lawyer. It seems like you will need to get a court order to return your son. Whether or not this may be filed in California depends more on the facts, such as when your son left to live in Arizona, where he lived before, and how long. Also, the biological father will need be involved.
Contact an attorney immediately.