The ruling on one who vowed to fast three days of every month but was unable to fulfill it due to illness

A woman fell ill with a severe illness, so she vowed that if she were healed, she would fast three days in every month. When she was healed, she fasted, but then she fell ill again after several years and was unable to fast. Does one of her sons fast on her behalf, or does she feed [people] for each day?
If her illness is not expected to end, she must feed one poor person for each day. She feeds three poor people, and no one fasts on her behalf; because a living person is not fasted for; rather, fasting is done on behalf of a deceased person;By the saying of the Prophet (peace and blessings be upon him): (Whoever dies with fasting owed, his guardian fasts on his behalf).