Can a woman be considered a Mahram for a woman she is not related to for purposes of traveling or sitting with others?
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 breastfeeding 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 and blessings of Allah be upon him) said, "A woman does not travel except with a Mahram." (Narrated by Al-Bukhari and Muslim)
The Prophet (peace and blessings of Allah be upon him) also said, "A man is never alone with a woman except that Satan is the third." (Narrated by Imam Ahmad and others from the Hadith of `Umar (may Allah be pleased with him) with an authentic chain of narration)
To learn more, review these responses:
Is the Father In-law a Mahram?
Is the father of one’s ex-husband a Mahram?
Is the wife’s sister considered to be a Mahram?
Is My Husband's Stepfather My Mahram?
And Allah knows best.
