It is impermissible for a man to sit in privacy with a non-Mahram (not a spouse or an unmarriageable relative) woman. This is because it is authentically reported that the Messenger (peace be upon him) said:
No man stays with a woman in seclusion except Satan will be the third one present.
(Part No. 17; Page No. 151) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.