Lucretius handles the problem in Book 3 of De rerum natura with a series of arguments that, whatever people may say, they do in fact generally fear "Tartarus and death", and (again, whatever they may say) this is true as well of people who claim to be materialists and say "that they know the mind to be composed of blood, or even of wind if that happens to catch their fancy" (p. 69). Lucretius thinks that this is obviously posturing for public consumption, and that if we look at their practice in adversity, we see clearly that people act in a way that is most reasonably described as fearing death and the afterlife:
For the same people, though banished from their homeland, driven far from the sight of other human beings, branded with stigma of some foul crime, and afflicted, in a word, with every kind of tribulation, continue to live.
Indeed, such people often will still engage in sacrifices to the underworld, etc.
However, there are further reasons, Lucretius thinks, for thinking that dismissal of the afterlife is just a show. There is a more indirect reason in looking at how well "avarice and blind lust for status, which drive wretched people to encroach beyond the boundaries of right" (p. 69) and "envy that before their eyes another possesses power" (p. 70) and other bad behaviors are often explained by fear of death and Tartarus; it is a general blight. Lucretius does not develop the full character of this argument, but one way to interpret it is as trying to argue that (1) this is a very general fear that people clearly often have difficulty overcoming even when it would be in their interest and according to their moral principles to do so; (2) the supposed reasons why people claim they lack such a fear are relatively superficial opinions (in contrast to Epicureanism, which is a philosophical discipline one practices throughout the whole of one's life); (3) and therefore it's implausible that such a superficial remedy could thoroughly cure a problem which the evidence tells us runs very deep.
We have the claim made by this person that they don't fear death and the afterlife because of their materialism, or whatever, but we have evidence (from the first argument) that even people who make claims like that are often seen, in their practice, to act contrary to what they say, and we also have evidence (from the second) that their explanation for why they don't fear death and the afterlife is entirely inadequate. Unlike the Stoics, the Epicureans did not regard rigorous abstract demonstration as an important goal, instead insisting on the importance of evidence rooted in sensible experience. Thus the issue is not whether there could be some hypothetical situation in which this line of thought could turn out wrong, but what our sensible evidence says; due to Epicurean epistemology, Epicurean arguments are framed in such a way that counterexamples are not easy to build against them -- they have to be real cases, not hypothetical ones, and cases where the question can be determined on the basis of actual experience. (One of the things that is very interesting about this argument.) It wouldn't be impossible to find such a case -- after all, Epicureans like Lucretius think they have the evidence of experience that Epicureanism cures the fear of death and the afterlife. But it would have to be an actual case.
[Quotations from Lucretius, On the Nature of Things, Smith, tr., Hackett (Indianapolis: 2001).]
