What relationship does Charlie have with Bill? Is this a healthy student/teacher relationship?

This relationship is murky. Although Bill seems like that cool motivating teacher every student likes, he blurs the lines between teacher and friend. Bill says the right things to really help Charlie,

"You're one of the most gifted people I've ever known. And I don't mean in terms of my other students. I mean in terms of anyone I've ever met."

I think, however, that the author gives us a vague unsettling feeling about how freely the two interact together.