but from the coral wiki it only states "Once the player has reached level 8 of friendship, they will receive a letter from Sanchez Brothers Blacksmith, telling them that to show romantic interest the player must give them a locket that is newly added into their catalog. The locket costs 1,500"
https://coralisland.fandom.com/wiki/Romance
and this is my second run in the game, i even could give the locket to him without seeing the event before, this also happened to another character too, not only Waku, I couldn't trigger his event either...