The baby dying when the mother dozed while breastfeeding it
If the reality is as you have mentioned, your mother has to offer
the Kaffarah (expiation) of involuntary manslaughter as she caused the baby's death. The Kaffarah is to free a believing slave. If she can not afford this, she has to fast for two consecutive months (sixty days).May Allah grant us all success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.