Legal Question in Wills and Trusts in California

Not only is a POA given by a person who is not mentally competent invalid, unless it is expressly designated as a "durable" POA and has essential language to validate that, it expires once a person who gives a valid POA later becomes incompetent. Your daughter could get in big trouble if she tries to exercise the POA, and depending what she did with it, she could be charged with elder financial abuse. She needs to be told to cease all activity under the POA, and you should review anything and everything she did under it, if anything, for signs of misuse of your mother's assets or credit.

If your mother really does need someone to take care of her affairs, and she is already mentally incompetent to do that, you (or your daughter if she is actually responsible and well meaning) need to obtain a conservatorship over your mother. If you think your daughter has engaged in misconduct under the POA, you need to contact Adult Protective Services for help, and get yourself named conservator which will give you the power to act on your mother's behalf to try to undo anything your daughter has done under the POA. https://www.co.merced.ca.us/1480/Adult-Protective-Services