What is the ruling on obligating an eighteen-year-old girl suffering from a mental illness and mood swings to wear the niqab, when she insists on wearing it?

Upon reaching puberty, a girl must cover her entire body when going out or appearing before non-mahram men, in compliance with the words of Allah Almighty: "O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments." Relatives must help and encourage her to fulfill this obligation, in compliance with Allah's command: "And cooperate in righteousness and piety." Thus, the niqab is a religious requirement, and there is no choice in matters that are religiously ordained, as Allah Almighty says: "It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair."