Seclusion With an Unrelated Woman is Forbidden

Question :

There is a certain degree of carelessness among some example, a man came to the house of his friend and he did not find him; and so the wife began to talk to this man who had arrived (the friend of her husband), and she admitted him in the sitting room and brought coffee and tea for him. Is this permissible, bearing in mind that there was no one in the house except this wife?

Answer:

It is not permissible for a woman to permit an unrelated man to enter the house of her husband during his absence, even if he is a friend of her husband, and even if he is trustworthy and dependable. This is because this includes seclusion with an unrelated woman, and it has been reported in the Hadith:
"A man does not sit alone with a woman, except that Satan makes a third."
Likewise, it is forbidden for a man to ask his friend's wife to let him in and serve him, even if the man is confident of his own trustworthiness and religiousness, due to the fear of Satan whispering to him and joining them.

It is incumbent upon the husband to warn his wife against admitting any man unrelated (to her) into the house, even if he is one of his kin, in accordance with the saying of the Prophet :
"Beware of entering upon women."

They said: "O Messenger of Allah! What do you say regarding Al-Hamu?" He replied:
"Al-Hamu is death (i.e., it may result in ruin for the family)."

Al-Hamu refers to the husband's brother or one of his kin, therefore it is even more necessary in the case of others.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Pages 121-122