If by "graphic" you mean "illustrated", Barefoot Gen is a manga series written about Hiroshima from survivor Keiji Nakazawa.

If you mean "graphic" as in explicit, then that will be tougher to track down- Japan recognized almost 450,000 Hibakusha, or atomic bomb survivors, and I'm sure many of these have written about their experiences. Do you have a date when she would have read it?

This isn't an essay, (it occupied an entire issue of the New Yorker and was later published as a book), but it is probably the most famous work about the Hiroshima bombing: Hiroshima by John Kersey. It covers the accounts of six witnesses of the bombing and aftermath.

http://archive.org/stream/hiroshima035082mbp/hiroshima035082mbp_djvu.txt