Classical migraine can start at any age. There can be substantial loss of vision in the early stages. This symptom can be the first one knows about it. Shock could easily cause the other symptoms the captain suffered.
This is not the place to ramble, but will go over the issues on the medical section if needed.
There is a type of migraine that causes sudden blindness in one hemisphere. Totally black on one side of a central vertical line.
In my opinion, although very rare to begin with no previous history, the danger is real enough to consider when deciding on the minimum qualifications of the only other pilot on board.