@Nakanowatari
During the time the web novel was up on syosetu there was a some complaints when it started to go down a certain path. It was revealed that the childhood friend betrayed him because she was afraid of the monster stalking him. The plagiarism accusation comes from the fact that said monster is based on a urban legend of an eight fool tall woman who stalks little boys she fancies. Although it's hard to call it plagiarism since it's an famous urban legend retold in numerous ways.
Either way it wasn't all that well received so it was taken down and in the process of being rewritten when the author moved to Alphapolis.
It's connection to Arifureta was never anything more than wild rumors at best.
BTW, said tall woman stalking crossed worlds to find him, they fucked like rabbits and he killed her because his bodily fluids are poisonous. But she ended up being reborn as SD Navi in his eye sight. Yea. Ridiculous. The storry actually improved and kept its tone after being rewritten.