What is the ruling on one who says that it is impermissible for the the husband's father to look at his son's wife, assuming that this does not make him a mahram to her?

A son's wife is permanently forbidden to the father, due to Allah's saying: "and the wives of your sons who are of your own loins." Therefore, she is among his unmarriageable kin (mahram), and it is permissible for him to see of her what a man sees of his unmarriageable kin. Whoever says otherwise is mistaken.