The legal liability of Air Traffic Controllers in the United States is a complicated issue but the final analysis is going to focus upon whether the actions by the Controller(s) violated their duty of care as set forth by Federal Law and FAA Directives.
The primary function of ATC is to prevent collisions between aircraft and collisions between aircraft and vehicles and other obstacles within the operating area (airport confines) and terrain.
Those responsibilities vary from VFR and IFR and the type airspace involved.
In this particular case....the accident aircraft was being operated VFR not IFR.
At no time did any ATC Controller issue a clearance specifying a minimum flight level to be maintained.
All Clearances given instructed the Pilot to operate within Visual Contact with the Surface and outside of Cloud.
The Pilot is the final authority re control of the aircraft and is compelled to comply with ATC Clearances except for Emergency Situations.
Under FAR Part 135, that responsibility is shared by the Operator's employee who is tasked to oversee Flight Operations but the final authority is the Pilot.
There was no emergency...there was no assigned Flight Level...and other than requiring the aircraft to remain clear of certain Control Zones and out of cloud.....there was no route of flight clearance issued by ATC.
The Pilot did communicate with ATC about planned flight paths and did receive replies from ATCconfirming receipt of that information.
The Operator needs a better Lawyer....one that understands reality and Federal Aviation Law.
Attempting to sue the Government and any ATC Controller is just a waste of time and paperwork.