Literally anything with the main protagonist being an Npc (in a game). Original or fanfiction, male or female, doesn't matter. Preferably longer than 20k words.
Since he's asking for a recommendation, it would be wiser to recommend a novel and why than simply redirecting him with a tag, no?
Legendary mechanic's main selling point is he reincarnated into the game and then gets to issue quests himself exploiting both game knowledge as well as the players, lol.