Well, from a technological standpoint, there are only 2 solutions to this.
Either A. He's faking it, or...
B. As you know, the controller for a robot (Or RC Flight Sims, in this case) sends a certain transmission; a transmitter. The RC Flight Sim plane is a Reciever, which recieves the signal, and then responds to the transmission accordingly. With a bit of techno know-how, you could take the reciever out of an RC Flight Sim and hook it up to a computer, either via USB, or hardwiring it directly into your computer. This would then send the signal to the reciever, as usual, but would then correspond it with the PC, much like a game controller.
But, wouldn't it just be cheaper to just get a game controller?
TL;DR, read Gropaga's post.