If the guy who is teaching has payed his dues to the graffiti game, i dont see why he couldnt teach it. When you get older, and realize that you putted so much efforts, time, money, problems with justice etc for this graffiti game, some people want to have some things in return .
Hey, he's teaching, getting paid, for a thing he loves, why not? We dont even know who is this writer anyway... maybe he bombed more than all of us.
But if this writer has stared graffiti 1 or 2 years ago, or is an unknown writer in his city...it would be another story...