A 82-year-old woman with a CFS of 5 and moderate dementia has been admitted to hospital. She has a baseline Barthel Index of 14/20. On admission, she scores 14/20. After 10 days in hospital, her Barthel has dropped to 8/20. She now needs help with feeding, transfers, and mobility that she did not need before. What is the SINGLE most effective intervention to prevent further functional decline during her hospital stay?