A Woman Does not Act as a Mahram for Another Woman

A woman cannot be a mahram for another. The one who is
considered mahram is a man that a woman

cannot marry due to blood relations, such as her father and her brother,
or a man related to her due to marriage, such as

her husband, her father-in-law and her step-son, or a man related due to
breast feeding, such as her father from breast

feeding and so forth.

It is not allowed for a man to be in private with a woman he is not
related to nor can he travel with her. The Prophet

(peace be upon him) said,

“A woman does not travel except with a mahram.”

This was recorded by al-Bukhari and Muslim. The Prophet (peace be upon
him)

also said,

“A man is never alone with a woman except that Satan is the
third.”

This was recorded by Imam Ahmad and others from the hadith of Umar with a
sahih chain.