The flight was a solo nav-ex.
The instructor was found liable mainly because in the training record he had indicated several areas of "carelessness" particularly relating to lookout, situational awareness etc.
The instructor authorised the flight with no expectation that the flight would be completed, because of poor weather on one sector. He had presumed that the student would divert back to Prestwick. However, the training record also indicated that the student was not good at following instructions.
It was accepted that the student did not have the ability to perform a 180 degree turn on instruments and get out of cloud if he entered inadvertently, despite having completed 90 hours instruction.
You can find the complete opinion at
www.scotcourts.gov.uk , then go to opinions and search on flying instructor.
As a consequence the instructor was found 50% liable.