Corporate pilot reported a ground safety hazard involving a helicopter jet wash while parked on the ramp.

Narrative

Aircraft were parked very closely to each other on the east ramp. There were two helicopters that had arrived; and were boxed in by multiple other jets. One of the helicopters began a hover taxi and completed a 180° turn and flew over top of a small piston; single engine aircraft that was started behind it; and then proceeded to fly over the cockpit of our parked corporate jet which we were preparing for an owner flight; and then flew within an estimated 25 feet of the tale of another parked corporate jet. The downwash from the large helicopter nearly flipped the small single engine; piston behind it; and created an intense rocking issue for our aircraft. The rotor of the helicopter nearly struck the tail of the corporate jet; parked immediately next to us. I confronted the crew of the helicopter; as well as the lineman who had parked him. I recommended that the operation was unsafe due to the close proximity of the aircraft and the strength of the downwash on the Jets and piston that they had flown over. The crew stated that the FBO lineman had directed them to perform that operation; and the lineman indicated that he was unaware of what the helicopter pilot was going to do. I spoke with the crew of the other helicopter and ask them to taxi with tower over an active taxi way; instead of flying over top of the parked aircraft as the previous helicopter had done. When I talked to the supervisor of the linemen; he seemed unconcerned and acted like it was not a big deal. The helicopter personnel were completely understanding and agreed that it was something they shouldn't have done. The FBO staff made no such indication that they saw it as a problem. I'm very concerned by the operations taking place at this FBO; the apparent inexperience and uncaring attitude of the staff; and the lack of attention to safety.