Should the young man's father disclose what was seen of pornographic images of the fiancée on his son's computer, or is concealment obligatory?

The young man should be advised gently to remove the image. If he does not comply, he should be threatened with informing someone who can rebuke him. If he persists, it is preferable to conceal his fault as long as he is not openly defiant, because Islamic law encourages covering a Muslim's faults. If the intention is to inform the young man's family about the girl's situation, the young man must first be advised that this girl is not suitable for him due to her dissatisfaction with her religion. If he is not convinced, do not inform his family about the truth of the photos. Instead, tell them that she is not suitable for their son, for the sake of concealment and to prevent him from marrying her.