There is a condition, headshaking syndrome, that I think is related to photosensitivity. A Guardian mask, or any one that offers UV protection can help. I have only heard of such things and not experienced it for myself, think I am kinda happy about that, but I have also heard that this is not always an unmanageable condition nor is it all that rare. Worth a chat with a vet about it as it would be a shame to pass up a good horse if it can be managed reasonably. Good Luck to you!
Sure there's right and wrong but mostly there's just a whole lotta different.