GMAT Critical Reasoning Practice Set 3

Clark must have known that his sister Janet and not the governess pulled the trigger, but he silently stood by while the jury convicted the governess. Any person of clear conscience would have felt terrible for not having come forward with the information about his sister, and Clark lived with that information until his death 30 years later. Since he was an extremely happy man, however, I conclude that he must have helped Janet commit the crime.
Which of the following assumptions must underlie the author's conclusion in the last sentence?