Adam was not deceived. He did not believe what his wife said to him to be true,
Lewis is here partly quoting 1 Tim 2:14 ("Adam was not deceived"), but even the (probably non-Pauline) author of that verse does not say that Adam disbelieved Eve after she was deceived. Even sticking to Milton, we know that Adam suspected what Satan said about the fruit to be true, or hoped so (IX:930-939), partly because he did not know its nature or the nature of death; he also believed Eve's report that Satan had eaten the fruit. In all this Adam and Eve were deceived.
