Depending on how much of the tea she drinks that is 300mL per mug or 900mL per day, that isn't so bad. Are her lips cracked or dry, if you gently pinch the skin on the back of her hand does it stay where you left it, is she not weeing, these are signs of de-hydration. If there are no signs of de-hydration then I wouldn't worry so much. Obviously if she were going to gym and sweating a lot it would be another question. If there are signs of d-hydration then contact your Doctor, she may need a drip to restore her fluid levels. For Mum one day at the Ambulatory Care unit with a canular in her arm was enough to get her drinking, she didn't want to go there again.
For Mum we gave her a change with Cranberry and Raspberry which was well received, now we alternate, Cranberry only and Cranberry and Raspberry. Because of false teeth issues a large full fat milk shake is her breakfast.
If you can keep a record of fluids in and fluids out, you then have evidence if you need medical advice.