Ah, can't recall the exact chapter, but it's right when Califa and Kaku are debating whether they should eat the DF, and the DF-using weirdo is shrieking not to bring the DF near him because the devil inside him will come out and fight the devil in the fruit...he's corrected that it only works like that if one eats the fruit. Then they go on to say something about how the powers work so I wasn't 100% sure if it's always lethal, but it sounds like it's Not a Good Idea, at least...