Get her in cooler conditions - the heat may simply be too much for her (or she could be ailing from something else, which makes her less able to withstand the heat). Give her some electrolytes. In a pinch, use Pedialtye (spritz on cold watermelon or other fruit she likes and refrigerate after opening. gatorade may also be used in a pinch).
I was worried about my oldest hen, 1-2 years old; well feathered, panting mouth open eating but not eating; she even stopped laying. Maybe it is the high heat and humidity. A big change from the cold and rainy spring. I will also try the cold water bottle trick and see if that helps. And to think I almost skipped over this thread. Thanks OP for asking!