"Don't betray me, I wish to be your friend." (Twain, 299)

Hello old friend, and here we are. You and me, on the last page.

...Sort of kidding. I'd apologize for the Doctor Who reference but, honestly, you're the one that needs to apologize if you don't watch it. I'll try and save the rest of my snobbery for later and cut to the chase now, anyway. This is the last entry for Huckleberry Finn! How excited are you?
(Don't answer that truthfully. Miss Watkins reads these.)
Starting with chapter thirty here, but not really seeing as nothing too interesting happens in it. The King and the Duke have a short, stupid scuffle because one thinks the other backstabbed him but after some booze all is forgiven, as is the True American Way™. Also a good example of some real friendship, right? Forgiveness and all that.
Chapter 31 is where things get interesting, though - and it only took 31 chapters! Basically, what happens it the King sells Jim to finance a drinking spree. This is a major betrayal to me. I mean, the ultimate way to ruin a runaway slave is to sell him back into slavery and that's exactly what happens here. Not cool.
When Huck finds out what happened, he has a serious crisis of conscience that didn't really surprise me. I've been skeptical about his relationship with Jim from the beginning, so the fact that he still subconsciously thinks of him as "property" only strengthens my belief that this is not a true friendship. It lacks trust and acceptance even after all this time.
Still, Huck does finally resolve to rescue Jim even if it sends him to Hell. The farm he's accidentally directed to by the Duke actually belongs to a relative of Tom Sawyer's because that isn't a totally outlandish and nearly impossible coincidence or anything! Neither is Tom showing up right after Huck does, apparently, seeing as that's exactly what happens. Shenanigans inevitably ensue. It's a lot like the beginning of the book, when the boys lazed around the (spoiler alert: dead) widow's house daydreaming and pranking the slaves.
Eventually, Tom and Huck manage to free Jim for about twelve hours. It's Aunt Polly that comes to his rescue this time. Thank goodness. She ties the story together neatly, declaring both the Widow Douglas and Huck's pap to be dead, therefore both of our main characters are free! When Aunt Polly expresses interest in adopting Huck, however, he immediately runs away again. I don't know how to feel about this in terms of what it means for Huck and Tom's friendship that he could leave him behind so easily the second time, but who really cares, right?
As I'm sure you've already figured out, I chose to focus on the unconventional examples of friendship and betrayal in the text. And I know this is late.