The reason in which Reki and Langa fell apart was their concept of skating changed. It's because Reki concept of skating was having fun and that was Langa's until the introduction of ADAM in which then the concept of skating he had was turned into speed and thrill. That slight change lead to a crack in their relationship in which progressively got worse leading to the fight.
It is not Langa's fault, nor Reki's it's the lack of communication between the two leading to the rift. But they are realistic teenagers and that happens; neither side is to blame.
I would like to state that Reki is a very complex character, he lacks jealously and strives for improvement. He understands Langa is better than him naturally and wants to see his friend improve instead of fail. It shows his true nature, and when he challenged ADAM, he was doing it for Miya after seeing him distraught from ADAM's words. He wishes to raise people up. He lacks being selfish and wants to see others improve a quality most teenaged boys do not have.
The custom board of Langa's is another showing of the thought and effort he puts into improving his friends, he takes all the parts Langa needs and makes it into a skateboard that best works for him and his snowboarder like qualities. Reki is a skateboard design genius by taking in c…