Nubian, if I drive my car into a bridge support that is just clear of the road (but was compliant with all the building regulations at the time it was built) is it the fault of the builder?
No, of course not, it is my fault because I misjudged the clearance (for whatever contributory reasons, rain, confusing road markings, not paying attention) - that is the root cause of the accident.
As with the helicopter, the second aircraft could have been parked there all day long without being the cause of an accident - the mishap only occurred with the introduction of the landing helicopter.
The primary and root cause was the pilot of the landing helicopter landing too close to the running one - she made a misjudgment, just as if she had taxied into the side of a hangar or a lighting stanchion.