If you have physical evidence that she stole the money (and that it wasn't a gift, or earned income for being a care taker or live-in helper), you may want to contact your local police or prosecutor's office: what you described sounds like theft from your father. If she was receiving food stamps or other public assistance benefits at the same time and not reporting her other "income" from your father's home, you could contact your local or state benefits office that distributes Food Stamps and or other public assistance benefits such as unemployment insurance. Fraud is basically hidden lying, cheating, and/or stealing. If she intentionally under-reported her sources of income, she may have committed benefits fraud. Your local district attorney's office can assist in describing the possible penalties. Be careful who you allow to have access to your elderly father - if he still has bank and credit accounts you may want to put "fraud alerts" on these to prevent further attempts to steal from him or use his identity to open new "joint" accounts/lines of credit. If you suspect further mischief, you may want to consult with an attorney to protect your father's interests.
This is a general comment to your posting and is not meant to constitute legal opinion or advice. There is no attorney-client relationship in this matter. If you think you need legal advice or representation for this matter, you should consult a licensed attorney in your jurisdiction.