Legal Question in Real Estate Law in California

Yes. The divorce decree is not binding on the lender who made the loan to both of them counting on both of them to repay. They can collect from either or both of them, unless they do a trustee's foreclosure, in which case they only get the house and can no longer enforce the debt.

To add to the previous answer, since the loan is in both your mother and father's name, if the lender forecloses, both of them will have a foreclosure on their credit report. Your mother can add an explanation to her credit report, but it will still affect her score and ability to find a new mortgage for some time.

It is quite rare for a lender to go through a judicial foreclosure. In a non-judicial foreclosure (trustee's sale), the lender waives its right to a deficiency judgment. Thus, it is unlikely that she will actually get hit with a monetary demand as a result of this default.