(My source: http://coveredpearls.wordpress.com/2011/09/15/a-woman-does-not-act-as-a-mahram-for-another-woman/)

Question:
Can a woman be considered a mahram for a woman she is not related to for purposes of traveling or sitting with others?
Response:
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 (sal-Allaahu `alayhe wa sallam) said,
((A woman does not travel except with a mahram.))
This was recorded by al-Bukhaaree and Muslim.
The Prophet (sal-Allaahu `alayhe wa sallam) 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 Hadeeth of ‘Umar with a saheeh chain.
Shaykh Ibn Baaz
Fataawa al-Mar.ah