Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow has dilated pupils and appears to not be able to see.
There are several reasons for the eyes to dilate.
Fear, pain and the release of adrenaline can cause pupils to dilate, but once he was back home and comfortable again they should have returned to normal. So either his pupils aren't dilated due to fear or pain or he is still very stressed or painful, or there is another reason.
Cats that are hyperthyroid (a tumor of the thyroid gland that releases high levels of thyroid hormones and thus makes the kitty feel on edge) could cause dilated pupils. If a cat is hyperthyroid for a period of time we can see secondary hypertension and bleeding into the eye. That can lead to loss of vision due to retinal detachment or retinal bleeds.
Cats can develop glaucoma and that can lead to loss of vision and dilated pupils.
Cats with kidney disease can develop secondary hypertension and bleeding into the eye. That can lead to loss of vision due to retinal detachment or retinal bleeds.
If it were one eye then a tumor in the eye could be responsible.
Less commonly we can see uveitis (inflammation in the eye), which can lead to red eyes, with inflammatory cells infiltrating the eye which can lead to loss of vision and dilated pupils. These cats have red eyes and may have cloudy looking eyes. This can be due to a viral or bacterial, parasitic (toxoplasmosis) or fungal infection or it can be idiopathic (unknown reason).
Primary optic neuritis (inflammation of the optic nerve that connect the eye to the brain can cause blindness and dilated pupils.
There are drugs, such as epinephrine, atropine, and some pain medications that can cause dilated pupils, but they should not lead to blindness and if he has been home for a few days they should be out of his system and his eyes should be back to normal or getting better.
Why was he hospitalized? This may be a complication of the disease process he was being treated for or a new problem.
At this point your fellow likely needs to be rechecked.
Please let me know if you have any further questions.