Just send this link.
Then you will just need to accept the cancellation.
Cancel an order
I assume they have paid. If they don't respond/use the link, you can still cancel as one of the reasons for cancelling a paid for sale is 'buyer asked to cancel'. This will trigger a refund, but no 'black mark' for you.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)