I find nothing to suggest that WA state has any kind of safety inspection law so, no, there are no laws that say one cannot sell a car that is unsafe to drive.

Besides, unsafe is only an opinion and no matter how many places he goes it will still only be an opinion, not a statutory defect.

That the frame is corroded could have easily been seen if he'd have had the car inspected by a mechanic before buying, or even looked under the car himself.

Your friend is welcome to take it back where he bought it and "ask" for a refund but I'm pretty sure I can guarantee that the visit will not please him.

If the dealer won't do anything, he is welcome to take the car to a collision repair shop and have them weld in braces to strengthen the frame and then he might get a few years out of it.