@ThatOneScizor I'm pretty sure mints only change the effect of the nature, not the nature itself. For example, if he used an Adamant mint, while the effects of the nature would go from +SpD -Spe to +Atk -Spa, the nature itself would still be Sassy. And if he would trade the dog back to Alola, since mints don't exist there, the game would use the effects of the original nature - Sassy.