Duffy - now playing for Brighton - played with Stones for Everton's reserves, and said that Moyes leaving the Toffees was the best thing for Stones, as Roberto Martinez moulded him into a ball-playing defender.
"You could tell straight away he was a very good player. The best thing for him probably was Moyes leaving," Duffy told 'The Argus'.
"He developed under Martinez into the kind of player he has become, played him at right-back. Moyes wanted more of a defender".
Duffy also said that Stones was one of the major factors in his own departure from Everton as first-team opportunities were limited, but there were no hard feelings.
"That's one of the reasons I had to leave. He was that good and he was obviously ahead of me. There are no hard feelings about that. I hope he goes on to do well," he added.