Whirly I think you're on to something. Modern aircraft are designed to be flown by average people. Considering that flying is equal parts motor skill and intellectual endeavour, anyone who is able bodied and of normal intelligence should be able to learn to fly competently (competently, not brilliantly!). The only obvious exception I can think of is the 'mentally unstable'. I mean, if you're coocoo then obviously you're out.
So, assuming that your 50 hour student has not given you reason to believe he is, in fact, insane, you should probably review your training, identify the specific shortcomings in both this gentleman's style of flying and your style of teaching, and make adjustments where neccessary. Oh, and stop calling him ****-for-brains, even behind his back.
Having said all that, I'm glad he's your student and not mine, ha!