So I'd handle this in this way: if they want to change the date of their booking request, they have to cancel the original one and request to book the correct dates. THEY have to do this - if you do it, it will block the dates.
If the card is not working, they need to use another card or contact AirBNB for help. You can't take the payment directly for a variety of reasons and there isn't anything you can do to help.
Lots of people outside the US use credit cards 😉 And lots of people from outside the US book on US AirBNB listings with cards from outside the US. And vice versa! Lots of people with US cards book nonUS listings just fine! Not sure what's going on here but it's not your problem 🙂