You would get better and more answers if you post this in the Washington forum near the bottom of the index page.
That being said, police generally won't return property they have seized except to the rightful owner. For the piddling things in the defendant's pockets at the time of arrest, they will just assume that's the arrestee.
However, for items of value, especially those bearing serial numbers that can be tracked, they would be irresponsible not to make sure that you are the rightful owner before giving them to you.
As far as whether the initial seizure was legal, that could depend on when the officer seized the firearm. If he took it into custody before cuffing your friend, he can claim "officer safety." Once your friend was cuffed, there might be a case that he no longer needed to seize the firearm for "officer safety." However, after taking your friend into custody, it would be reasonable for him to safeguard the firearm. Leaving it in a car at the side of the road, at best to be picked up by a friend (who may or may not be able to possess or carry it), at worst to be stolen, would also be irresponsible.
The full answer to your question would depend on details that you have not provided and Washington law.
I am thinking, though, that your friend has no case.