Eye injuries can be tricky. One of my old pony mares got a piece of hay slivered in her eye once and the vet had to come and cut it out. She had all sorts of antibiotics and ointment creams that we had to put on it for weeks. She had to wear a fly mask 27/7 to prevent infection and insects irritating it. She was not very happy at all. But THANKFULLY she made a full recovery. As long as you are consistant with the treatment the horse should heal well. Just make sure you call the vet if you see any changes or swelling.
The vet also told me to feed lots of treats when treating her eye. You do not want to make, touching around her eyes or face unpleasent which could lead to head shy issues.