What is an assumption loan?
You said that the division of property is in the divorce decree so your mother should send a registered letter and a copy of the divorce decree to the mortgage lender and if she knows the ex husband’s address and phone number so they can start hounding him, if they haven’t already. Send a copy of your letter and divorcee decree to all three credit bureaus and tell them to make this a permanent notation on her credit reports. The lender made the suggestion of assumption as a way of hopefully transferring the debt to someone who could make the payments.