“No man should be alone with a woman unless there is a mahram (male relative whom she could never marry) with her.” Narrated by al-Bukhari, 5233; Muslim, 1341.
An-Nawawi (may Allah have mercy on him) said: If a non-mahram man is alone with a non-mahram woman, with no third person present, this is haraam according to scholarly consensus.
End quote from Sharh Saheeh Muslim, 9/109
As-San‘ani (may Allah have mercy on him) said: This hadeeth indicates that it is haram to be alone with a non-mahram woman, and there is consensus on this point.
End quote from Subul as-Salam. 1/608
Secondly:
There is nothing wrong with a woman allowing a non-mahram man to enter her husband’s house when he is absent, as long as two conditions are met: