I would hate to say that this is a simple case, but (to my inexperienced mind at least) it revolves around a single issue:-
Should any Instructor, when admitting that they had no expectation of a student finishing a XC due to deteriorating weather, actually sign a student out to commence the exercise?
The ability, learning difficulties or flair of the student should be entirely inconsequential here, as should any records of training, criticisms or otherwise. Had the student been told that they could not be signed out for this flight due to bad weather, the abilities of the student, the records and comments about them would never have been an issue.
I'm afraid I would hold the Instructor entirely responsible for this tragedy, as it is his authorisation of this flight that directly resulted in the death of the student. Negligence, pure and simple.