I was wondering at first too if he was Murakami's Max Perkins or Gordon Lish or something. Nervertheless, in his latest essay about Yasuhara appeared in Bungeishunjyuu ia big Japanese magazinejhe denies the fact saying he didn't edit or help his writing although Yasuhara praised his writing and encouraged him to write more. In other words, Yasuhara was Murakami's few important friends ithat's what he saidj. It was when there were a lot of people who didn't admit his work, so he said he thank him for that very much. He also said he's still not sure why and when Yasuhara's attitude chaged into attacking Murakami violently. In the essay, he seemed more disappointed and wondered than angry and furious. At the conclusion, he wrote that it's deplorable to know that he laments not only Akira Yasuhara's death but also something else.
Sorry about bad english. Did it help you understand the situation?