horrible example
If a man who failed school because he never paid attention in class told you to focus in school, it's still sound advice whether or not he's a hypocrite. Even though he couldn't follow his own advice, it doesn't mean his advice isn't credible.
In fact, you could probably even say that since he didn't pay attention in school, he has first-hand experience of what happens if you don't focus during class therefore it could make his advice even more convincing.