Here is what I think...
1) we still really only have the passengers side of the story. Why do I say this? Because outside of a court of law the cruise line would be crazy to say anything "negative" about the couple right now. It would only make them look worse. The cruise lines now know that in the current environment they can never win a PR
battle - especially with people with medical challenges.
2) The cruise line talked to the woman's son before the husband returned, who said "My mom has dementia." They probably learned more than enough to justify putting them off the ship based on that talk. I do understand that dementia comes on stages and giving the husband the complete benefit of the doubt I could say "he didn't realize how badly she would react to not knowing where he was."
But on the other hand - if the son knew, and the cruise line talked to her and made their own determination (It does not take long to determine whether a dementia patient is at stage 1 or stage 10), they apparently decided the husband was very wrong to do what he did even one time - which means he is a "bad actor" and they didn't feel comfortable giving him a second chance.
Maybe he was hoping she might disappear on that cruise.
Also bad for the husband, in THIS country in such a situation there is a good chance the gov't will step in and mandate that he put her in a full-time care facility, that she cannot live at home anymore. Then he wouldn't be able to afford any more cruises. They may have done him a favor.
But the bottom line is that cruise lines are NOT adult baby sitters, even for people with dementia or Alzheimer. It is not their responsibility. He should not have made it their responsibility (which he admitted he did right in the interview).