It sounds like a central artery occlusion - did they mention that? They have have also checked for a condition called Giant Cell Arteritis which can cause CRAO.
Central vision is sometimes spared if they have separate artery (called a cilia-retinal artery) leading to the macula which is the part of the eye that is responsible for central vision.
If it is a CRAO (central retinal artery occlusion) then there's nothing much they can do, except to check that there are no co-exisitng conditions such as high blood pressure or high cholesterol levels. No treatments have been found to have any affect as the tissue damages so quickly.
It Must be incredibly hard for him, but people do adjust to having vision in just one eye. He needs to ensure that he does as much as he can to look after the vision in the good eye; so no smoking (smoking is really bad for eyes), good diet, exercise, good control of diabetes.
Its worth getting the other eye checked to see if your husband needs to wear a correction for the good eye.
He also needs to inform the DVLA, but they allow people to drive usually after 3 months, which is when the vast majority of people have adapted. Your husband will need to learn to move his head more.
Having written all this though, I don't know for 100% certainty that it is a CRAO, but you could have a look at the discharge papers or call your GP.