I think the best approach would be to understand why they have this belief in the first place (which you have not stipulated) and approach it from there, rather than attacking the life-vs-death logic directly.
This question could come up when considering whether to have children or to not have children. If the state of living is more valuable on a philosophical level, then you might be pressured to have children because of that. But if "life" vs "not life" on a philosophical level are equal, then it would be equally acceptable to choose not to have children.
From the Letter to Menoeceus, it seems to me that the philosophical attitude regarding the value of life vs death are equal:
[126] "But the many at one moment shun death as the greatest of evils, at another (yearn for it) as a respite from the (evils) in life. (But the wise man neither seeks to escape life) nor fears the cessation of life, for neither does life offend him nor does the absence of life seem to be any evil."
And, if you are already alive, then the idea is to not shun this life.