Prove Huckleberry Finn isn't racist?

To understand that Huckleberry Finn is nor racist, you must understand the nature of Satire, which is saying the exact opposite of what you mean in order to provoke a visceral reaction.

If you read HF literally, it does sound racist. But it is carefully constructed in order to convince the reader that Huck's beliefs are false. The key is the section when Huck thinks over his own beliefs about racism. Thinking about ownership and property rights, Huck decides that he is willing to be "evil" as regards all the laws of his time, in order to be true to his inner sense of righteousness. Although he still labels himself as "bad" the reader rejoices in his moral development. In spite of all the forces against him, Huck has reached the right conclusion.

It is this moral understanding, on the part of Huck and the reader, which means that the book is, far from racist, a great work in favor of racial equality. Twain uses the simple thoughts of a simple-minded boy and the one-on-one fact of a father's love for his child to tell us that we are all the same, no matter what the color of our skin.

Your friend, unfortunately, is still at a developmental level where he does not understand satire or irony. He reads only the surface, and so he misses a lot. Try to explain to him that a work of literature means more than the sum of its parts. Reading about Jim missing his child, about Huck being willing to be a "bad" person in order to do a "good" thing, the reader is moved to see Jim as fully human, and those who aid his reunion with his family as taking the only morally correct path.

Sorry, I can't read you chapter an verse on where Huck has this revelation. I read the book many years ago, and I remember it because it taught me that "right" and "accepted by society" are not the same thing.

I honestly don't think that Huck Finn was meant to be a racist book. This is for more than one reason. One reason being that It is still sold today. They wouldn't sell it if it were racist. It was written in another time, when blacks weren't as respected as whites. Huck Finn has not gone down in history as a racist book, and I honestly do not think it is.