I just beat this game a few hours ago and I was wondering what does the coke can do? The only person you can use it on is that camp dude next to the machine, but it's not nescessary to do so. I am curious to know if it affects the game at all. I even got the two endings. Awesome game by the way.
It does nothing at all. That wrestler dude just takes both the cans although you'll need only one. So its just a filler for the inventory.
Giving The Camp Crusader the coke was the original way to unlock the alternate ending. If you did so, there was a dream sequence right before Brain wakes up in Part 3, in which CC tells him of the connection between Smiley Joe and Mr. Nice Guy.
Then ... I realised that didn't make any sense. I kept the coke bit in, though ;D