The reason I wouldn't return his cards, even if 100% identifiable, is that the player voluntarily folded. Once you elect to fold anytime before the showdown, IMO it is binding, whether you do so in turn or out of turn. An interesting question that might bear clarification at the June Summit.
I really see the sequence here as two separate actions:
A: Player calls. He's obligated to make the call good.
B: Then player voluntarily folds his cards and that is a binding fold.