Once I had a hen with something that sounds like what your hen has, it made a big swelling of her eye and she couldn't see out of it. With gloved hands and strong intestinal fortitude I gently squeezed at the base of the swelling and got a big goober of gunk to come out from under her inner eyelid. It looked like a big yellowish caterpillar and even had a little rounded indentation at the end where it was resting on her eye.
Her eye remained a bit swollen for the rest of that day, but the next day she was FINE. It was one of the grosser things I have ever done, but it made me feel like a miracle worker, to have restored her sight!
I've done this with a few other hens over the years, haven't always had such great success. I think a lot has to do with just how severely the bird is affected before you get around to doing this. I think it is caused by upper respiratory infections that get in their sinuses & up into their eyes. But I'm no expert on this, it's just my theories.