How to refund a gift from someone else to them?

They would need to request a refund. https://support.xbox.com/en-US/help/subscriptions-billing/buy-games-apps/refund-orders

If they can't do it there, they would need to contact support and speak to billing. http://support.xbox.com/contact-us

The funds from the gift card are automatically deposited into your account, so you can't gift them to someone else. You can gift funds to someone in your family, but that is disabled at the moment.

Gifting is region locked so there's no way to make is to work for them. You could try requesting a refund and then buy it somewhere else where you can get them a UK code. Or give the code to someone else.

https://support.xbox.com/en-US/help/subscriptions-billing/buy-games-apps/refund-orders

If you bought it for them through Xbox gifting, the code is region locked. You can give the code to someone else who does live in the US or try to refund it and buy the game somewhere else that would work in Canada.

Gift cards are non-refundable, the only thing you can do is give or sell to someone else if you haven't used them. Bear in mind you can only use them in your region, so if you do give or sell them, it also has to be from someone in your region.

Also, please do not post your phone number on a public forum, as anyone can see it.