You will need to inform the credit card companies of the situation, and they should write off the debt. Your mother will likely receive 1099s reporting the write-offs as income to her, but given her situation, this is unlikely to result in any taxes due.
Harry S. Margolis practices elder law, estate, and special needs planning in Boston and Wellesley, Massachusetts. He is the founder of ElderLawAnswers.com and answers consumer questions about estate planning issues here and at AskHarry.info.