After examining the photo at length, my impression is that hyperplasia is occurring on the conjunctiva, building up thickness and gradually over time pushing the lower eye lid downward. It's probably benign and could have started as an irritant in the eye causing tissue to grow over it. It's like when you get a tiny splinter on your hand and a callous forms over it. It's just my own wild theory, not a scientific or medical diagnosis.
As long as the hen is behaving normally, I doubt there's anything you would need to do. If she isn't scratching the eye, it's probably not giving her any discomfort. If she does scratch at the eye, it may be because it's dry. Lubricating eye drops would ease the discomfort. In fact, regular administering of eye drops might be a good idea to keep it clean so she doesn't get eye infections.