Have they ruled out an ocular migraine? I've known two people that have had those.
One was prone to regular migraines, but this was different. About 5 years ago, she temporarily lost her sight and there was numbness in one of her arms. She thought she was having a stroke. The other had never had a migraine, but had two ocular migraines when she was pregnant (30 years ago). The first time, she was shopping at a department store and lost her sight and strangers stepped up to guide her to a place to sit down. They did not get a headache, and neither of them has had an ocular migraine since.
I would quietly inquire about this without him knowing. I wouldn't want to get his hopes up and them have them quashed if it turns out to be something else.
Prayers up!
Ocular Migraines Explained - AllAboutVision.com