I am not familiar with this particular incident but am curious about your definition of "duty of care" Mr Eagle (moderator) ?
Duty of care is one of those terms that can and often is employed in abstract fashion to mean anything you want it to mean. The responsibilities of a Commander are usually enshined in the operations manual and in turn governed by legal statute.
If a crewmember obtains permission to disembark the aircraft downroute and then subsequently fails to rejoin it prior to departure time it clearly raises a number of serious concerns but I cannot find any section in statute or operational instructions that make such a situation a breach of any "duty of care".
From a Captains perspective there would naturally be a concern regarding any missing crewmember and that would involve the airport ground representative ( presumably at management level) and the police if necessary. Ordinarilly the onus would be on the crewmember to rejoin the flight in a timely manner or use their common sense to communicate any difficulties in so doing. Nothing has been suggested here to indicate that wasn't actually done ? The captains responsibilty is then to the company and that would be to ensure that the operation from both a safety and efficiency standpoint is not unduly compromised. It is not an unreasonable hypothosis to suggest that a flight could depart one crewmember down where that crewmember is not absolutely necessary to the operation. This would no doubt be balanced in consideration to the "duty of care" to the paying customers on board as well as those that any delay might affect on later flights. The costs to the company as a consequence of any such delay would also be valid. At many of the resort airports in Europe and indeed at airports where an airline has a significant representation, there are often multiple flights on the same day and any subsequent repatriation of an aircrew member is unlikely to present much of a logistical problem. Notwithstanding that, the crewmeber would be repatriated at whatever cost by the most expeditious means.
I am not sure where the assumption came from that this was a young person but even so they were still presumably a reasonably intelligent adult ? Foreign country assumes the individual wasn't already a citizen of that country ? Presumably they were an E.C citizen and since they hadn't commited any crime the point is fairly redundant. I find it hard to believe that such a situation stemmed from any malicious intent on the part of the other crew or the Commander so this is hardly "nasty".
Palma airport is one of the busiest holiday airports in the world. Slot delays here in the summer can have major commercial repercussions for a company. I would certainly question the wisdom of allowing any crewmember to wander far from the aircraft for non urgent matters in the course of an ordinary turnaround. However I can well understand not allowing commercial damage to result from such a situation given that sensible procedures and proper liason and reporting were carried out by any commander.
As a footnote I would also point out that very short shrift is allowed to passengers who fail to board the aircraft on time. They are often left to fend for themselves without due regard to their alternative repatriation. Assuming such passengers are also adults, the fact that they may be "young" or in a "foreign country" is also normally of precious little concern.