One of my sis' Gerbils is being really lethargic and has some sticky dried discharge on one of his eyes. I cleaned it off with a wet washcloth but it still looks extremely lethargic and "off". It seemed fine the other day. I know nothing about Rodents...
What kind of bedding is your sister using for him? Is he making any clicking noises or breathing heavily?
He might be having a allergic reaction to something in the cage or he could have a respirtory infection. Make sure he's getting plenty of water and food. If he's not eating she may have to feed him by hand. The best thing she can do for him is take him to the vet. He may need to be put on medications.
