In the first LUK SIU FUNG story (THE MYSTERY OF THE GOLDEN ROC KINGDOM'S TREASURE), Fa Mun Lau was established as Luk Siu Fung's best buddy. Certainly, during that first story, it seemed that way, and Fa Mun Lau had almost as much facetime in that story as the title character himself.
After that first story, however, Fa Mun Lau's presence in the LUK SIU FUNG novels was sporadic at best. He had only a very limited appearance in the second story (THE EMBROIDERY THIEF) while introducing the character of Yip Goo Sing, and was entirely absent from BEFORE AND AFTER THE DUEL. He reappeared in some of the later LUK SIU FUNG stories, but was never again featured as prominently as he had been in that first story.
During those intervening stories in which Fa Mun Lau did not appear or had only limited appearances, Luk Siu Fung had numerous interactions with his various other friends...Sai Mun Chui Sheut, Honest Monk, Chu Ting, See Hung Jak Sing, Sit Bing, etc., etc. He did not necessarily seem any less close to these individuals as friends as he did to Fa Mun Lau. Was Fa Mun Lau really closer as a friend to Luk Siu Fung than were any of Luk's other friends?