You may have to wait until she goes to roost this evening, or just don’t bother. Break the 25mg diphenhydramine in half or into 1/3 tablet. Dosage is 4mg per every 2.2 pound or Kilogram. Have you gotten any new birds in the last few weeks? It sounds more like she may have a respiratory infection, such as MG. Usually I recommend cleaning the eye with a clean and warm damp wash cloth, and applying either Terramycin eye ointment, Vetericyn, or plain Neosporin twice a day. Your parents may want to buy some Tylosin powder here, and treat her or any other one that shows those symptoms:
https://jedds.com/products/tylosin-powder
Dosage is 1 teaspoon in a gallon of water for 3-5 days.
https://jedds.com/products/tylosin-powder
Dosage is 1 teaspoon in a gallon of water for 3-5 days.