A couple more possible take aways: your climber, and the group, exercised admirable discipline and judgement, in stopping / convincing to stop, with so little distance to the summit. Not always easy to do.
Second climber "disoriented, spacing out", different story, possibly was more serious than "mild AMS". I would look up the signs and symptomology of HACE (high altitude cerebral edema) and compare to your climber's condition and experience. Don't know whether any medical follow-up is indicated now, but others on this board will.
In any event, congratulations on your summit, but more important congratulations for getting everyone down safely.