If you listen to the ATC recording, around 15:50, they instruct the helicopter to watch for traffic, specifically this flight, and clears them for virtual separation.

It's the helo's fault. They likely misjudged the plane due to assuming it was a large jet but it was a regional jet, so it was way closer than they thought it was.

It's a tragedy, but I don't see how it would be ATC's fault. But that's just my 2 cents.