Is it permissible for a divorced man to marry his ex-wife who has returned, knowing that she has abandoned the hijab and changed from how she used to be, and he seeks to guide her back to the straight path; would he be sinful if he marries her while she is in this condition?

The principle in Islam is that no soul shall bear the burden of another, but a person bears the sin of their shortcomings regarding those under their guardianship. So, if you fulfill your duty by advising your wife, enjoining good upon her, forbidding evil from her, and obliging her to wear the hijab, you will not bear any of her sin. It is advised not to marry her unless she commits to wearing her hijab.