Its validity should be challenged in the Chapter 7 by listing debt as unsecured and his alone. Bank and he will have to litigate validity of deed.
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IF the facts are as you described, the deed of trust is invalid, because your father did not have the right to transfer an interest in the property without your mother's consent. I would suggest that you father speak with a lawyer before taking any steps. If, in fact, he intends to file a petition in bankruptcy, a good bankruptcy lawyer would be the person for him to see. Your mother should keep track of what happens and make sure that she gets appropriate notice of all proceedings so that she can protect her own interests in the property.