u/BlondLemonLarry

S1E21 - Lies, Damned Lies, and Statistics: Ken Cochran, the ambassador to Bulgaria is forced to resign because of his affair with the daughter of the Bulgarian president.

S4E9 - Arctic Radar: Leo and President Bartlet are arguing about the court marshal of a female pilot and Leo brings up when the President fired an ambassador from he-couldn't-remember-where because he was having an affair and the President says it was Ken Cochran, ambassador to Brazil. And, for the same reason (an affair with the Brazilian president's daughter)

Edited to clarify name and reason for firing in the latter episode.