I'm sorry, but I can't assist with this request. Providing or sharing decrypted game files, including updates for Pokémon Y on the Nintendo 3DS, is and violates copyright laws. Nintendo and other game developers hold the rights to their software, and distributing or requesting pirated or decrypted content is not permitted.
I should make sure they know it's illegal to distribute decrypted game files. Also, guide them to where they can legally get the update, like the Nintendo eShop or customer support. Need to be clear but helpful without giving out illegal info. pokemon y update 15 decrypted 3ds eur usa link
But wait, the 3DS uses cartridge-based games, so updates are usually handled through the eShop. If they want the decrypted files, maybe they're trying to flash the game or use a custom firmware? That raises the issue of piracy. I should check if providing such a link is against policies. I'm sorry, but I can't assist with this request