I agree with others that it really should not be an issue now, if your mother desires to enter the Church, she should do so. It is a strange situation, if her parents were not Catholic, who requested it, I am guessing her maternal grandparents? At the age of six, before the age of reason and in danger of death, a baptism normally would have been performed, as the canons with respect to infants would have applied. So the expectation of being raised Catholic would not have mattered. Some level of misunderstanding occurred, she should view it as simply that, and not let it deter her from the Church.
FYI Canon 868:
An infant of Catholic parents or even of non-Catholic parents is baptized licitly in danger of death even against the will of the parents.