Aircraft interaction
There has been much speculation recently about the "what if's" regarding the passage of these two aircraft - had one gone higher or lower, etc. There is one fact about aviation that seems to be overlooked in all of these discussions, and that is how aviation works in the first place. For an aircraft to fly, the air pressure above the wing must be lower than the air pressure below the wing. Likewise, for a helicopter to fly, the pressure above the rotor must be lower than the pressure below the rotor. So, there is by definition a pressure differential that is required for all of this "magic" to work. (I refer you to the Merion, Pennsylvania mid-air collision of April 4, 1991, where the collision occurred because the helicopter pilot was attempting the fly underneath an aircraft with a reported landing gear issue, in order to perform an assessment of the landing gear issue.) As a result of this pressure difference requirement, if a helicopter (or another aircraft, for that matter) were to fly beneath another flying aircraft, the downdraft from the helicopter would suck the airplane down (by virtue of reducing or elimination the higher under-wing pressure) into its own rotor. This would happen without either pilot doing any flight adjustment of their own. By virtue of the now altered pressures surrounding both aircraft, they would migrate towards each other all on their own. Now if the helicopter were to fly above the aircraft, the same thing would happen, but in reverse - the higher under-rotor air pressure would be reduced or eliminated by the actions of the aircraft's wing, and again, the two aircraft would be drawn towards each other entirely on their own.
I am reminded of an instance in perhaps the late 2000's where a U.S. military aircraft was buzzed by a Russian MIG. The MIG approached the U.S. aircraft on the right side, and passed very near to the aircraft, and very near to the aircraft's right wing, passing just above the right wing. This action caused the downdraft from the MIG's wings to press downward on the U.S. aircraft's right wing, causing a reduction in lift, and causing the U.S. aircraft to begin banking to the right. The MIG pilot asserted that the U.S. pilot did this on purpose, but the fact of the matter is that the interaction of the two pressure systems interacting that actually caused this behavior. Fortunately, in that case there was no collision, but there very well could have been.