Praise be to Allaah.
Ibn Qudaamah (may Allaah have mercy on him) said:
“A kaafir is not a mahram for his Muslim female relative. Ahmad said concerning a Jew or Christian whose daughter becomes Muslim: ‘He cannot travel with her, he is not a mahram for her.’ Apparently he meant that he is not her mahram for the purposes of travelling. As far as looking is concerned, she does not have to wear hijab in front of him, because Abu Sufyaan came to Madeenah at the time when he was still a mushrik, and visited his daughter Umm Habeebah. She folded up the Prophet’s bed so that he would not sit on it, but she did not wear hijab in front of him, and the Prophet (peace and blessings of Allaah be upon him) did not command her to do so.” (al-Mughni, part 7, Ahkaam al-Khutbah).