We're not sure he qualifies as a mentor, but Willy Wonka is definitely a guide. He literally gives everyone a tour of his factory. Plus, in the end, he gives that factory to Charlie, which completely and utterly changes our hero's life. And in that sense, maybe he is a bit of a mentor. What are mentors for if not to better the lives of their protégés? Plus, don't forget: someone's going to have to teach Charlie how to make all those confections.
Everyone's favorite Grandpa comes in a close second as a possible candidate for Charlie's guide/mentor. He's Charlie's constant companion throughout the book, and he is also Charlie's main source of information about the chocolate factory before he wins the Golden Ticket. Grandpa Joe seems strangely knowledgeable about the factory, and isn't shy about sharing this knowledge with his young grandson. But we can't help but wonder, how in the world does Grandpa Joe know all this stuff, anyway?