Was a friend's mother's death preventable?

A friend of mine had a mother that he was supposed to take care of. She had dementia and ended up dying of a drug overdose since she forgot she already took her meds. I can't understand why he didn't manage her medication when she already had had problems taking them too often. I asked why he didn't take control of her meds and he said it is because she would throw a fit about it. I think that dealing with a fit is less important than someone's safety. Am I wrong to be upset and see this as a preventable death and that my friend bears some responsibility?