Your daughter in law cannot change her name in the passport as per Indian laws because she is an American citizen and her passport is USA passport and not an Indian passport. Her passport is governed by the laws of USA, hence she has to abide by the rules of USA in this regard, thus, better approach an attorney in USA for more clarity on this.
It is not mandatory for a wife to change her maiden name after her marriage, at least not as per Indian laws.
She can change her marital status in her USA passport by submitting application and complying with the necessary requirements accordinlgy.
Her marriage certificate is another proof of her marital status.