What is the ruling on a man looking at his son's wife with lust and trying to seduce her, and what should the son's wife and the son do in such a situation?

The son's wife is forbidden to the father according to the text of the Quran. If he looks at her with suspicion or attempts to seduce her, he has committed a grave sin and it indicates a perversion of his nature.

The wife must treat him as a stranger, observe hijab in his presence, and not allow him to be alone with her.

As for the son, he must denounce his father's action kindly. He may shun him if that is a means of reforming him, to the extent necessary, while maintaining communication through other means.