There's no telling how she did it. Horses are the most creative animals in injuring themselves ever.
My appaloosa mare once managed to cut her eyelid half off. As in, it was sliced from one edge to just past the center of it, so the upper eyelid was hanging down. The vet gave her sedation to make her sleepy and stitched it up, no harm no foul, healed up well. But despite searching for hours, I never found anything that could have cut her eye.
Glad to know she did heal up.