It's not hard to figure out who "the host" is in the lightly fictionalized new section added to the paperback edition of Morning. From Page Six:
The new section contains a story about someone caught up in a highly publicized controversy and a television show that he knows is going to end badly for him... He starts taping all his phone calls.... After getting publicly eviscerated, he tapes one with "the host," where she checks to make sure he isn't "going to hurt himself" and reveals some startling information about herself.
The show is clearly Oprah Winfrey's daytime talk program, where she first praised then excoriated Frey. The "startling information" is a bit trickier; it could relate to persistent rumors Winfrey is involved in a lesbian relationship, it might be something else entirely.
Most likely: We should listen to the attention-hungry author when he tells us "the book is fiction." That's one thing he's never lied about.