The grieving atheist cannot provide any reason why he grieves, or why he (rightly) respects the grief of others. For to grieve the death of such a young man is implicitly to affirm the reality of the soul. Man is embodied, to be sure; but what is embodied is a soul, capable of memory, reason, and love. To grieve the loss of anyone then is to lament the departure of a unique being, whose mind and heart have touched our lives in spontaneously beautiful and inimitable ways. To grieve is to travel even beyond the lost life of a loved one to the origin and source of the love we have known, and there to register our gratitude. To grieve, therefore, is to affirm that there is a higher source of value than 'the selfish gene' - there is a God, who is absolute truth and goodness, the very possibility of knowledge and love.
It's interesting that you can see the glimmer of understanding that atheists are just folks much the same as everyone else. At least Dunbar acknowledges atheists do grieve and respect the grieving of our fellows, even if he thinks they have no reason to. This is a far cry better than refusing to acknowledge atheists have the same feelings as believers. Christina's response is almost perfect because it is so emotional and not coolly rational.