How can one respond to a woman who claims that the niqab is not obligatory, citing the opinions of some sheikhs, while insisting that uncovering the face is sinful?

The most authoritative opinion is that it is obligatory for a woman to cover her face and hands along with the rest of her body if she fears temptation, especially with the spread of corruption in this era. This is based on Allah's saying: "O Prophet, tell your wives and your daughters and the women of the believers to draw over themselves their outer garments." [Al-Ahzab:59], and on the Hadith (definition) of Aisha (may Allah be pleased with her) regarding drawing the jilbab over the face when riders passed by, and her Hadith about the early Muhajirat women covering their heads. Therefore, a Muslim woman must wear a full and modest hijab (definition) and avoid places of temptation.