CL-650 flight crew reported a helicopter had hover taxied directly overhead their aircraft while under marshal instructions from the line service agents on an FBO ramp.

Narrative

Aircraft X was positioned on the FBO ramp at ZZZ approximately centered on the FBO and about 20 yards out from building. We were just about to begin before start checklist when we observed a Bell 407 helicopter coming into a hover to our left. I paused the execution of the before start checklist because of the possibility of FOD damage to the engines. After a very short hover we observed the FBO personnel signaling to the helo to come right (toward us). The helicopter started to slide right and gaining a small amount of altitude. They crossed over the top of us at approximately 20 feet above us and then executing a tail turn to back into its landing spot to the right of us. I immediately called on the FBO frequency requesting to talk to a supervisor. I then met someone" on the ramp and explained my frustration of the event that just took place. I told them that was an extremely dangerous maneuver and that it should never have happened. I then called our company and talked to duty manager and explained what had just happened. During the event our fight controls did not move. Before departing both pilots of Aircraft X completed another preflight looking for any signs of damage or FOD. None were found. We departed the ramp with no further incidents and had a normal flight. The ramp personnel at the FBO should never have asked the helicopter pilot of to do what it did. The pilot of Aircraft Y should never have accepted their signals and should have never been in the mindset that it is okay to hover taxi over a parked aircraft clearing it by only approximately 20 feet. As a former helicopter pilot with over 1500 hours of flying experience in rotary wing aircraft; I never did anything remotely close to what this pilot did. I am quite certain if I had; there would have been some swift discipline assigned to me."