Oh , I don't know. Sometimes you just have to give up, throw in the towel and dump it.
If he was honest about the trade in to the dealer then I have no problem with him trading it in.
Let the dealer deal with the problems and hope you have better luck next time.
Personally, I can never afford to take the loss doing this , so I get stuck.
My brother once bought a brand new pickup. It ran fine until he hooked to his 5th wheel camper and headed out on the interstate. Then it would go into "limp Home " mode and he would get stuck driving 40 mph to the nearest dealer. He took it to 4 dealers in 3 different states. He thought they had it fixed. Then the next year when he headed out on vacation it happened again. They had this big vacation planned with no way to change it and his wife was in tears because they had it all planned out. He found a dealer with a brand new truck, traded in his one year old truck and dumped it. The headed out on vacation one day late but, had a good time.
Sometimes you just have to say "Screw it" and move on.