50Dropped by
the drop condition is a name
Nothing about the Guide himself changes. You simply need a world where he rolled the name Andrew. Renaming him, or renaming somebody else to match with a resource pack, does nothing, because the check runs against the original English name rather than whatever is printed on screen.
getting it means killing him
Town NPCs shrug off your weapons, so you need an outside source of damage. Lava is the classic answer, though a deep pool will burn the hat along with him. Traps, drowning, Explosive Bunnies and a conveniently placed enemy all work. The tidy method is a Guide Voodoo Doll, which makes him hittable with whatever you are already holding and drops him exactly where you want the loot.
where the name comes from
Andrew is Redigit, the person who made the game. The hat is a nod at Luigi, and it belongs to a very small club of items gated behind one specific NPC name, alongside a cap from a correctly named Painter and a drop from a correctly named Steampunker.