My 91-year-old Mother's Short- And Long-term Memory Has Been Declining

My 91-year-old mother's short- and long-term memory has been declining for a few years, but there has been a seemingly big decline in the last couple months. She had a lumpectomy this week (day surgery). As part of the process, she had an IV line with fluid (to keep her hydrated) and electrolytes. When the sedative wore off about an hour after the surgery, she was more alert and was remembering short-term events really well. The effect lasted about one day. Can either greater hydration or more electrolytes improve short-term memory? If so, are there available over-the-counter electrolyte products that we should be trying? How much fluid should an inactive, 115 pound woman be drinking each day?