Her husband divorced her; is his father still a mahram to her?

Praise be to Allah.

There is nothing wrong with that, because Allah says (interpretation of the meaning): “Forbidden to you (for marriage) are: ... The wives of your sons who (spring) from your own loins,” [an-Nisa’ 4:23]. They are the wives of the sons, so if the son divorces her, she is still a mahram to his father. 

The mahram connection between her and his father remains in effect. That is similar to the case of the father’s wife; if the father divorces her, she is still a mahram to the son. Allah, may He be exulted, says (interpretation of the meaning): “And marry not women whom your fathers married” [an-Nisa’ 4:22]. If his father divorces her, she remains a mahram to his sons. The same applies to the wives of sons. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).