What is the ruling on a da'wah (Islamic propagation) page owner accepting the friend request of a person who has placed a picture of an unveiled woman as their profile background, and is this considered an endorsement of their sin or assisting their friends in sin?

Displaying a picture of an unveiled woman is a sin requiring repentance. However, it does not necessitate breaking off friendship with the person who does it. But if you mean accepting their friend request on social media pages, then you should first advise them. If they respond positively, then that is good. Otherwise, you should not add them, because doing so would be assisting in the viewing of prohibited images.