She might be dehydrated in the heat. Can you give her some water with electrolytes or some Gatorade? Do you have any vitamins that contain riboflavin? Here is a good article about Mareks:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
How old is your sick hen? Does she still lay eggs? Does she curl the foot under when she walks or have trouble walking? How often is she coughing? Are there other respiratory symptoms? Sometimes it is hard to diagnose a problem without labe work or a vet. Reproductive issues are common in hens...