This isn’t possible for a number of reasons, but the main one is that on the night in question (the night before disembark), the ship was out at sea.
The cruise line has already said he doesn’t appear on any of their video from the morning of disembarkation. (I’m not saying they’re necessarily 100% trustworthy, I’m just repeating what they said.)
And just for anyone curious: passengers get on and off the ship via long mobile gangways that are extended to the ship when it’s docked. Everyone is checked/scanned off as they leave. There isn’t really, for example, some secret exit they could use to sneak off.
(And just to chime in for cruises, I understand why some people find the idea loathsome, and I have no interest in 3-day party cruises, but I’ve been on some really beautiful ships with super nice and chill people. Unfortunately it’s a “you get what you pay for” thing, and I find that a lot of people who say they cruised once and hated it were on the cheapest/shortest cruise available to them.)
FWIW (which I’m sure isn’t much… lol), I went on a 3-day Carnival cruise out of Jacksonville, FL to Nassau, Bahamas and had a blast! I wouldn’t say it was any more of a “party cruise” than the 7-day cruise I took earlier that year to the Western Caribbean out of Port Canaveral. Of course, the 3-day cruise was less expensive and the ship was much smaller but only because it was a much shorter trip. They were more or less the same experience, though. They both offered poolside games, bingo and trivia, a day camp for kids, karaoke, talent competitions, a spa, casino, dance clubs, cigar bars, art galleries, comedy shows, theater productions, fancy restaurants, all-you-can eat buffets, 24/7 room service, and a huge atrium with a crystal chandelier.
Nevertheless, I get why some people have no interest in going on a cruise. They don’t like the idea of being out on the open water with no land in sight, the prospect of the dreaded Nora virus contaminating everything and everyone on the ship, or the size of the staterooms gives them claustrophobia. My husband is one of those people, but I wish he wasn’t.
In any event, I’m worried for this young man. Like many others have mentioned on this thread — there’s really no place to hide on a cruise ship and I can’t think of many hopeful outcomes with the facts that we have.