I am so sorry for your loss.
I am wondering if perhaps this was from a Sago Palm seed because normally macadamia nuts don't cause death. Here is a link about macadamia nuts:
Merck Veterinary Manual
And one about Sago Palms ... those things are horrible and do cause death. We have them all over in this area and I am thinking perhaps you do as well....
Sago palm poisoning cases increase - VIN