We know of indications that she was complicit in attempts to obstruct justice, and who knows what else she could be in serious trouble for? Or perhaps she's faced up to that reality and that she was going to need most of any book proceeds to pay her enormous legal bills and get on her feet again, perhaps once she left prison.
Right now, talk of a diary is pure speculation.
Talk of an eventual lucrative book deal will not be; Hicks is a walking diary. However, her book would not necessarily be a serious betrayal of someone who reportedly regarded her somewhat like a daughter, much less a trashy tell-all hit job.
Trump has a long history of getting along well with women he worked with, and of generally treating them well. Of course, a "feminine" demeanor and usually good looks were necessary to be accepted into his world, but many women have fit those requirements over the decades.
Otoh, I do remember that someone must have told Trump that trusted snake fable and that the people Trump gathers around him reveal a strong affinity for people who share his horrible character and lack of principles.