Cyclicflare - you can hear the whine of a Gazelle fenestron from miles away through double glazing but I don't think hearing him would have helped him assess the proximity of the Gazelle.
If the robbo pilot has his back to the Gaz and is in the cockpit with headphones on and awaiting start clearance I don't think he can be considered negligent or culpable in any way, shape or form.
The gazelle pilot has a clear view of the area, presumably knows he is down wind and that his rotor wash is ahead of him and still comes in close enough to bother the R22 - that is poor airmanship however you dress it up.