Once the helo said they had the aircraft in sight and maintaining visual separation ATC had zero responsibility for traffic separation at that point. Not trying to speculate, but I assume the helicopter had the aircraft behind in sight and never saw the one they should have.
Yes atc should’ve seen it coming after the fact but with the way these approaches and helicopter corridors are set up There’s so little time for mistakes.
This is why I'm putting it more on the helo pilot and less on ATC. ATC asked for visual confirmation of the CRJ, helo pilot confirmed. I am not with the NTSB though and am just an armchair investigator with minimal knowledge of the actual facts. Anyone taking my thoughts on the matter should take that into account.