If you are relating what happened and want to use a pronoun instead of repeating passenger:
"A sick passenger was disembarked. ?? / the passenger had complained
of severe stomach pains ... ."
then you can choose between "he or she" and "they".
There have been numerous discussions on this site about the singular they. There are some site contributors who adamantly oppose its use, whereas others, and I include myself among them, find it a good pragmatic choice in most spoken contexts where the gender of the person is either unknown, unimportant or to remain undisclosed.
Note, however, that the singular they is becoming more and more prevalent in British English in particular.
For example, I get a newsfeed from the Guardian newspaper with a link to an article by a different Secret Teacher each week. To maintain strict anonymity, this singular secret teacher is always referred to as they. Last week's introductory text contained the following sentence:
This week's Secret Teacher reveals how they and their colleagues are
struggling due to the onus put on teachers to oversee every step of
the behaviour process ... .
If you want to pursue the topic further, there are numerous entertaining and informative articles on Language Log about the singular they .