To add some colour to my last post, the minimum safe distance between helicopters operating on adjacent TLOFs is 1/2 the width of the largest helicopter in play. Assuming the discs of these helicopters is around 10m, that would mean at least 5m between rotor tips.
Now in a dynamic situation we cannot expect the pilot to accurately judge 5m. However, in the video it looks to me like the landing helicopter actual descended "through the rotor tips" of the stationary running helicopter. That is clearly not 5m or anything like. WHY? because poor knowledge has led to risky shift which has been in play for a prolonged period where nothing has ever gone wrong.
Compliance is the first building block of Flight Safety. Had that pilot known on that day that the minimum distance should have been 5m, this accident would almost certainly not have happened.
(Note I am guessing the rotor diameter cos I am too lazy to look it up, however, if its 4, 5, 6 or even 7m I think you can get the principle I am applying).
Concepts such as risky shift, learned behaviours and poor compliance are symptomatic of Company Culture. If I have learned anything in my time in aviation its that of all things that may need to be changed, the hardest of them to change is Company Culture.