I've known a few "weak" parents that put up with this kind of crap. They enable their adult kids to rule the roost. They fear confrontation and the criticism that follows if they throw the abusers out. Now as per the OP,their son is in his 40's,they don't understand that they helped to create and shape his behavior and abuse.

They need to get some backbone and tell his son he's moving out..capice?..