Putting together the different press reports it seems clear than Ms Cutland's health was causing her family serious concern. Several of them suggest the other passenger was a friend or relative accompanying her back to a town or city where a doctor could consider whether she was OK to fly home and/or diagnose treat her condition. All of that seems to be exactly what might be expected if a young student had gone off the rails on a field trip to a remote part of the world.
While it's interesting to contemplate how the door might have opened sufficiently to allow her to fall I don't see any other plausible scenario then that she jumped. Any plot executed on board or anything involving dropping somebody already dead or critically injured would face exactly the same difficulties. It's also clear there have been similar incidents in the past.