No, you can't move save files from one user to another. A fair warning, games you purchase from the Nintendo eShop are linked to the Nintendo Network ID (and the balances from a NNID on the eShop are shared between 3DS and Wii U) so if you bought any eShop games under his old account or any money is left on them, they will be lost if you decide to delete it. 
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Re: Save file for SM3DW question
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