The wife of a son of one’s loins is a mahram to his father, because Allaah says (interpretation of the meaning): 

“Forbidden to you (for marriage) are… the wives of your sons who (spring) from your own loins”

[al-Nisa’ 4:23]

If a man has a son who is married to a woman, his father becomes a mahram to her, and he may travel with her, be alone with her, look at her face, and so on. With regard to his son through breastfeeding, most of the scholars are of the view that he is like a son from his own loins, but Shaykh al-Islam Ibn Taymiyah rejected that and said: Breastfeeding does not affect the in-law relationship; the wife of a son through breastfeeding is a non-mahram to the father, so she should not uncover in front of him and she should not be alone with him and he may not travel with her, because she is a stranger (non-mahram) to him. Allaah says (interpretation of the meaning):

“Forbidden to you (for marriage) are… the wives of your sons who (spring) from your own loins”

[al-Nisa’ 4:23]

And the Prophet (peace and blessings of Allaah be upon him) said: “What becomes mahram (forbidden for marriage) through breastfeeding is that which become mahram through blood ties.”

A son’s wife is not a mahram to his father because of blood ties but rather through marriage. There is no blood tie between his father and her, rather she is his mahram through marriage. This is the view favoured by Shaykh al-Islam Ibn Taymiyah, and this is what I think is correct.