Did he use it while they were married? Can it be established that he was the one who used it and not her? If you feel he committed a crime, you should contact the police.
Please note that no attorney-client relationship is created by my answers to questions on the Avvo forum. My responses should not be construed as legal advice, regardless of whether I am admitted to practice in your state or not. I am an attorney in Miami Beach, FL, practicing foreclosure, bankruptcy, credit repair, debt defense, and aviation law. Please consult a lawyer in your state if you seek legal advice.
Contact your credit card company and assert the charges were the result of fraud. Then, if you are willing to, you should contact your local police department and file a police report asserting that you were the victim of identity theft and that your son in law was the party responsible.