Hi ,
Good day! My name is Vishal. I am an Independent Advisor and Microsoft user like you. I am glad to be able to provide assistance for you today.
She can use Minecraft gift card to purchase Minecoins or modification on Nintendo switch but she will have to redeem her gift card through Microsoft redeem page but Make sure she signed with the same Microsoft account that she used on her Nintendo switch for playing Minecraft.
Once she redeemed a gift card and she used to purchase Minecoins. the Minecoins will be added to her Microsoft account. The Minicoins should appear in her Minecraft for Nintendo switch.
I hope this information will help you. Please let me know if you need any further assistance.
Regards,
Vishal