An old guy without a huge amount of helicopter experience (est 1400 hours in the report), in a confined area without his glasses on giving illegal instruction to another old guy with negligible helicopter time in poor light conditions - what could possibly go wrong with that?
I think the eyesight factor is a major cause, in part because he couldn't see the trees ahead but primarily because he didn't notice the drift during the vertical climb - basic scanning to a lateral marker would have picked this up straight away.
5 S recce is great but picking good markers is even more important.
If the 'passenger' in the LHS was flying the departure, all the major obstacles were on the other side of the aircraft where the instructor was sitting and his inexperience would make a drift in a vertical climb quite likely without verbal or physical intervention from the instructor.
If the instructor was flying the departure the his proximity to the obstacles would, in my mind, have led him to drift deliberately left in the vertical climb - presuming he could actually see the obstacles well enough.
Either way, a very avoidable accident with tragic consequences.