In another recent thread here I advocated in two bladed helicopters the blades be either tied down or at 3-9 o'clock position with the tiedown safely stowed and some took exception to that self imposed rule.
Application of that simple rule would have prevented this occurrence. Anyone that leaves an untied blade of any kind directly over the tail boom is asking for trouble.
That being said....the other aircraft is not relieved from the responsiblity of watching out for his rotor wash effects.
Being a former Chinook pilot....I do get a certain perverse satisfaction out of hearing about a Gazelle and a Robbie having rotor wash problems.