She fed her mother something harmful and she died
Question

                My mother told me not to cook a certain type of herb, and she said, “If you cook these herbs it will make me die, because I cannot stand to smell them.” Note that these herbs are permissible according to sharee’ah. And in fact after my mother and I ate some of these herbs for dinner, she died a few hours later. Am I guilty of sin for that? Did I have a hand in my mother’s death? Is there any sin on me for that?.
Praise be to Allah.If the situation is as you describe in the 
    question, then you have sinned because this was disobedience and bad treatment towards her. You are guilty of sin since you knew that your mother 
    was bothered by (that food) and that she had told you not to cook it. So you were sinning by doing that, and you were disobedient, severing the 
    ties of kinship and disobeying your mother. You have to pay the diyah (blood money) because your act is counted as semi-deliberate killing. You 
    also have to offer expiation (kafaarah), which is to free a believing slave; if you cannot do that, then you have to fast for two consecutive 
    months, i.e., for sixty days. And you must also repent to Allaah. 
We ask Allaah to accept your repentance and 
    ours, and to help us to do all that is good.