Hi @Matthew90,
That's frustrating that your guest is having issues with the payment. Your guest will need to contact Airbnb herself, as it's an issue with her account. This Community Help Guide explains the best ways to contact them.
Just to let you know, I have moved your post here to a new thread so that it doesn't get lost in another discussion and it is easier to find. I have also given it a title, please let me know if you want to change it!
I hope she can get this resolved!
Kirstie