That is a sad story. It appears that there were two things that happened either one could have prevented the tragedy. The students seat belt was not fastened and the canopy opened. Something is wrong here. Hopefully we will learn more.
Several years ago, near here, a man hired a Cessna 150 and pilot to take him up to photograph his house. While circling the house, the man undid his seat belt, opened the door and jumped out. It happened so quickly that the pilot couldn't stop him. It seems that he was getting a divorce and wanted to hit the house as a final gesture.
Once upon at time, I was doing a split S in my Stearman, While upside down I noticed that my seat belt felt loose. In fact it had come completely off. Note to self: Make damm sure that self maintains positive Gs until right side up !! This was one of those military style belts with the locking lever. It seems that I must have caught the lever with my jacket sleeve on my stick arm and never felt it. Solution was to reverse the seat belt. This could explain why some folks use double seat belts for aerobatics. I must say that this was a most scary moment in my life. Heck, it was an open cockpit. It didn't even have a roof to catch me like Bob Turner's Decalthalon.