Sell the car. If it's all in his name, then sell it and
pay off the loan with whatever you get for it. Then,
finish off the rest of the loan month to month. I
don't see any legal recourse here, though I'm not a
lawyer. Your father bought a car, then regretted it.
Unless you are trying to argue he was manipulated into
it? In which case, he would have to have some sort of
condition that put him in a position to be manipulated.
Or are you saying that they co-signed on the car? I'm
unclear, now that I'm re-reading your post.