Fulfilling a vow that is makrooh
Question

                My grandmother has vowed purely to Allah to fast the month of Rajab forever if her wishes come true. After they came true she fasted as she vowed. But she is old aged now and it is difficult to fast due to illness and weakness. What is the ruling on this? Does she have to continue fasting? Can she expiate instead and free herself from this vow.
Praise be to Allah.
    Singling out Rajab for fasting is makrooh, so this woman made 
    a vow to do something makrooh. If a person vows to do something makrooh, 
    then it is better for him not to do it and to offer kafaarat yameen 
    (expiation for breaking a vow). So she should offer kafaarat yameen by 
    freeing a slave, or feeding ten poor persons, or clothing ten poor persons, 
    giving each poor person half a saa’ of food, or clothes that are acceptable 
    for praying in. If she cannot do any of these three things, then she should 
    fast for three days. This is better for her, and she should not do what she 
    vowed.