Is it permissible for one who has vowed to fast Mondays and Thursdays voluntarily for life, with the intention of exception for days of legal excuse and illness, to change the wording of the vow to "whenever I am able"? And is she required to make up the exempted days? And what is the validity of a friend's statement regarding a vow based on a mistaken belief?

Scholars have differed concerning the phrase "I pledge to Allah" (أعاهد الله): The majority considers it an oath, while some hold that it is nothing unless intended as such. What we issue a fatwa on is that it is both an oath and a vow if one commits thereby to an act of obedience. The Malikis and Hanbalis consider it one of the explicit forms of an oath, and expiation becomes obligatory if one intended it as an oath or uttered it generally. The Shafi'is consider it one of the implicit forms of an oath, and it does not become an oath except with intention.

Based on what we issue a fatwa on, if you intended to fast Mondays and Thursdays perpetually, this is a vow that must be fulfilled. However, we see no objection to adopting the view of those who consider it merely an oath, given the hardship that might befall you. Therefore, you may suffice with an expiation for an oath. The belief in its obligation based on your mother's statement does not affect the ruling.