I agree with @OhEsme about her agent being likely to be still reeling. An agent takes on a writer because they’re genuinely enthused by their work, and they do often become close. They’ll have worked on initial edits together, and a pitch letter for editors, and been involved in the sale, or auction, if more than one publisher was interested, and the pleasure and excitement of it doing well.
It’s possible Jen Christie is an amoral cynic with £££ signs for eyes, obviously, but it’s equally possible she believed she knew Sally Walker well, and is just another in the line of people disturbed by the realisation that they didn’t.
I also agree, @Vroomfondleswaistcoat — if Chloe H points out inconsistencies in the supposed walks, routes, dates, the defence will be rearrangement for narrative purposes, and pointing to other travel writers whose work can’t possibly have been in any way an accurate account of their travels, like Patrick Leigh Fermor, whose A Time of Gifts, about the first part of a walk done when he was 18/19, wasn’t published till he was in his 60s. Or Bruce Chatwin, who said he didn’t see any distinction between fact and fiction.