It is indeed cruel to show people that they wasted their lives on some harmless delusion. However if their delusion motivated them to create something evil that ruined other people’s lives, then it is our duty not only to point out their delusion, but take action against their deeds.
Deng XiaoPing had the courage to face the colossal mistake he made during the first half of his life and took action to correct it. Why can’t Lu’s in-laws do the same?