Jeff sat in the cafeteria area at the airport while his wife, Gina, watched and commented on Sun and Jin. He seems to have little interest in observing others, a quality not beyond his wife. However, he does comment on "their" (i.e. Korea's) divorce rate, and how it is 20 times lower than America's divorce rate.
Though probably on a different flight, it is possible that he could have perished on Flight 815. Indeed, he did have an American accent, supporting the idea that he and his wife were on a flight headed to the United States.