What is the ruling on a husband who views nude pictures of women and men and asks his wife to participate with him, claiming there is no prohibition in that, and is the wife sinful if she participates?

Allah has forbidden letting one's gaze roam and looking at the 'awrah (private parts) of men and women. Pornographic films fall under this prohibition. The husband's statement that there is no harm in looking at nude people because they are not sacred is false, and you must not participate with him in this reprehensible act. The husband must repent to Allah and cease looking at forbidden things.