If there was no valid power of attorney in place at the time the mortgage was signed then the mortgage is invalid as it applies to your father. This can be a defense to the house being pledged as security to pay back the loan. Your mother will remain liable for the debt on the note but there will not be a security interest in the house. Your father can use this defense without prosecuting your mother however the lender can press charges for loan fraud if your mother presented them with documents purporting to be a valid power of attorney. If the lender never asked for an original power of attorney (which is unlikely) then they may be left with just a promissory note from just your mother which amounts to a personal promise to pay back money or an IOU. I suggest you consult with an attorney and bring all the loan documents with you before taking any action.
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I agree with Mr. Chertock. It is suggested that your family consult with an attorney as soon as possible. In a foreclosure action (a) defendant(s) only has so much time to file an answer to the action or to cite any defenses. This situation definitely calls for the advice of a licensed attorney in your state that practices foreclosure law.