My memory is that the French pilot is serving a considerable prison sentence for this.
If he HAD filed a flight plan, would it have been known to the helicopter pilot?
If the helicopter had known his flight plan, would it have helped?
Only radio announcements on the same frequency, or ADS-B both out and in could have assisted "see and avoid".
And "see" has faults. I only avoided a Pa28 200' above my C172, both descending, our radar blips fused, thanks to the very agitated ATC we contacted for approach.