What can we tell the nonbeliever about their eventual passing and its significance, to help them understand what can happen?

This is the most difficult of situations. Aside from routine conversations about the realities of life, when one is faced with a nonbeliever in their end-stage life experience, it is quite difficult to entertain unfamiliar and uncomfortable thoughts of what is coming. The nonbeliever has created quite a trap for themselves in denying existence of the divine and thereby dooming themselves to a future of nonexistence. This can be taken in stride by some, but most people find the whole idea deeply uncomfortable to contemplate and will put on a game face, but inside are quaking because they fear the nothingness. This is a universal fear because all, at one time or another, have experienced being in limbo and that memory is triggered with the advent of old age creeping up on them and a growing awareness and certainty their days are numbered. Consciously, of course, they are unaware of their inner knowing about this and would discount this entirely as laughable, but they are creating a dilemma that they will very likely re-experience a time in limbo yet again because they have neglected their spiritual development and their ties to the divine realm and will be woefully unprepared for their passing to make a full transition back to the light, where they came from originally. It will likely not be helpful to have this picture painted for the nonbeliever. They are in a state of fear already and giving them something else to be afraid of that is directly connected to the belief in the divine, whose existence they deny, is not the way to reintroduce the subject. It will seem to them as having two choices to experience, both of which are unworkable and highly frightening. The best you can do is to reassure the nonbeliever they will be looked after, and that is a good thing, and to be open to the possibility that their consciousness will go on, and should that happen, to simply summon the light and keep the focus on bringing the light, and that will bring many new experiences of wonderment and joy. It cannot hurt. They may scoff at this, but it will also plant seeds of wisdom within and it may be just enough that at the right moment they are able to summon this notion and make it work.