If in doubt, always call your vet or the Animal Poison Line immediately.
Avocados contain a fungicidal toxin called persin which is found throughout the fruit : flesh, skin, stone, and leaves. In dogs it primarily causes vomiting and diarrhoea. The large stone is also a serious choking and intestinal blockage hazard.
Call your vet if a large amount was eaten or if the stone was swallowed. A small accidental taste of flesh is less urgent.