Is a man who marries a Christian woman, whose religion does not obligate her to wear the hijab, sinful if she goes out and appears in public with her head uncovered?

A Muslim man married to a woman of the People of the Book is not permitted to allow her to go out displaying her adornments and uncovered, because this is an endorsement of her spreading indecency. He is commanded to prevent her from doing so, and he would be sinful if he allowed her. The 'awrah (intimate parts) of a free Dhimmi woman is like the 'awrah of a free Muslim woman. She must cover her 'awrah and refrain from displaying adornments that incite فتنة (strife/temptation).