Forgive me, I just didn't get it

I must be really thick.  There's a twist in this novel I did not get.  I'm talking about Forgive Me by Amanda Eyre Ward.

Nadine is a journalist attracted to dangerous places. Covering a story on drugs in Mexico, she is severely beaten and left for dead.  She wakes in her dad's house back in Nantucket.  After only a few days of healing, her doctor, with whom she has a quickie affair, hands her a newspaper which covers a story related to her past. 

Krispin and Sophia Irving, from Nantucket, are heading towards South Africa for the Truth and Reconciliation Commission hearings.  Their son Jason was killed there ten years prior, and now they have been asked to forgive the killers.  Nadine happened to be in South Africa during the time he was murdered, and suffered a simliar loss.  She needs to return to put her own demons to rest.  So she attends the hearings, helps free a young woman involved with Jason's murder, helps Sophia forgive the killers, faces the loss of her own loved one, and eventually grows up enough to maybe go home and commit to her doctor and his baby that she's carrying.  Phew.

Okay, this is the part I don't get.  Krispin for some reason hands Nadine Jason's diary while they are on the plane to South Africa.  We get to read sections of the diary in the book.  But the diary seems to be written by Nadine's son, and not by Jason.  Though sometimes it seems it's by Jason.  What did I miss?  Even after re-reading a couple of chapters as a review suggested, I didn't get it.

And I kind of liked the book.  Ward's spare prose captures an emotionally stunted woman at a crossroads, when she needs to make some heart-wrenching decisions about people who were very special to her at one time.

So could someone just read this book and explain it to me.