Just google "wood toxicity" and you will get plenty of university and wood research sites that give you great data on what is toxic and not. Walnut wood is not toxic but can be an irritant. Try these sites for documented info. http://ohioline.osu.edu/hyg-fact/1000/1148.html
http://www.cs.rochester.edu/u/roche/...isc/wood.toxic
No one had dropped dead from using one before and the juglone is not water soluable meaning that what he cuts on the board will not absrob juglone anyway. Mostly juglone is toxic to some plants.