Never seen it in real life. Only on tv and film ...... sometimes.( those high definition cameras seem to capture this) But that would be freaky to see it in real life.
Been a while getting back,but trying to find this in a medical way ,I found one condition.
Rieger syndrome .
It is a series of mutations that one does cause a vertical eye.
Here is the problem ,from what I can tell the slit is ongoing with a clouded pupil.
As well people with this have glaucoma.
This does not fit the description of what I looked at.
Honestly when this person is in direct light or irritated the eyes turn to vertical slits.
This person is a Christian and has been faithful for many years.
I have a reflection that occurs ,due to cataract lens that freak people out sometimes, lol it appears my pupils are twinkling ...i can't make this up!
I believe the slit is caused by the pupils contracting ,and a light reflection.
But I intend to do more research and get back to you when I find a solid reason.
Keep in mind this person has been to eye doctor ,but I cannot know if he had seen this happen.