If he was at a regional he could still be making $50K and be unhappy.

Very true. I have been a CFI for almost an year now. Now I love flying and love instructing. However, on a really busy day I could have my first flight at 8am and then when all is said and done I would be in my car driving home by 11pm. After a string of days such as these, you really do become exhausted and performance does indeed degrade. There were days I hoped for bad weather because I was plain old exhausted. Although there are no rest requirements, I think it is our duty as CFIs to take care of ourselves, have some rest periods and not be overloaded with scheduling. I am always in the belief that I want to fly as much as I can (brings me closer to that airline job) and also I am not lying when I say that I want to help people accomplish their goals as well. However, instructor burnout is inevitable sooner or later if one over works...So CFIs take care of yourselves, love your job, do what you can to love your job in the future too. And mostly have fun on the side. Eat healthy meals, workout, and do whatever else that makes you happy. Now I better get some rest :)