I'm not sure where exactly in Washington your friend lives or what pet stores he frequents that inform him of the illegality of Western Hognose snakes are, but I've lived in Southern Washington for 13 years and have kept Western Hognose for 6+ years and see them for sale on occasion at our local pet store. I've also been to the Puyallup reptile expo and have seen Western Hognose for sale there as well. Nobody that I have known that keeps or breeds Western Hognose has ever raised any issue about their legality.
Here's also an interesting
article that addresses this as well:
"6. You live in Federal Way and intend to buy a pet hognose snake (genus Heterodon)...
Hognose snakes are very mildly venomous and are a commonly kept species with no history of serious injury to keepers. They are native to the United States, stay fairly small and are widely accepted as safe to keep.
Like the mangrove snake, they are legal by state law and county/city code is ambiguous on their legality.
You are still in a legal grey area but you are less likely to encounter conflict and more likely to win a court dispute with little effort."