Have you asked them to consider contacting yahoo or contacting their customer support directly ? I know gmail accounts can be recovered in cases like these. I would suppose that Yahoo being a multi-million company would also have something in place.
Yes, account recovery is a very different concept from hacking, and it's the term you are looking for in this case.
Also, I'm pretty sure that asking about "hacking" an account would be illegal, and against the rules of this forum.