The Welsh singer scored a huge hit with her debut album Rockferry, which she worked on with Butler, scoring a Grammy Award and three BRIT Awards, but failed to replicate its success with her 2010 follow-up Endlessly.
She has since been largely absent from the music scene and Butler admits he feels sorry for his former protegee.
He tells Britain's Sunday Times newspaper, "She went off the rails and it all went pear-shaped for her. And maybe she brought all those problems on herself. But I always had quite a lot of sympathy for her, because she was young, from this tiny village in Wales, and she was just hurled into the fire. And everyone said, 'Don't act strange.' Why? Of course she was going to go off the rails."
Butler goes on to compare Duffy's troubles to the problems he suffered as a member of rock group Suede, adding, "When I was in Suede, I did the same to a certain extent; that was one of the times when I was closest to being a bit nutty, because it was a bit of a baptism of fire."