My grandmother accidentally purchased itunes gift cards thinking its pick and pay food gift card is it possible to get a refund

It is not possible to return an Apple Gift card.

Did she redeem the card?

Once you have redeemed your Apple gift card, you will receive a message:

• You have redeemed your code.
• $$ xxx,xx has been added to your Apple ID balance. Your balance is $$ xyy,xx.

See -> Check your Apple ID balance -> Check your Apple ID balance – Apple Support (UK)

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If you have Apple ID balance you can't use.

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•           Your Apple ID.

•           The refund amount.

•           Your current delivery address.

Click here for more support and assistance.