Unfortunately, if the property is in such a position that it is worth less than the value of the mortgage, and your mother is not able to pay the difference, there is not a lot I can offer to fix the problem.
The lender is entitled to enforce the loan as it is written, as I noted initially, there is no mechanism to force the creditor to change the loan terms or to accept a reduction in interest (this is true even if your mother were to declare a Chapt. 13 "repayment" bankruptcy).
If your mother is unable to make payments on the property, she can consider a "strategic default" where she pays little or none of her payments until the creditor finally decides to foreclose on the property. This will allow her to live in the home for a period of time without making payments on it, but eventually she will need to vacate.
Alternatively she can attempt to sell the property (either in a straight sale - which may be difficult as you say the property is underwater, or in a "short sale" - which would likely require using a short sale realtor specialist to help her negotiate with the bank).
I am very sorry, I do wish I had a different answer for you here, but I want to be clear and direct so that you can move forward appropriately.