@Sam351 Yes, the guest will get a full refund if they cancel it from their end (though under certain circumstances, such as multiple cancellations in a year, Airbnb might withhold their service fee). And yes, you'll be penalized if you cancel it yourself, unless customer service grants you an exemption under Extenuating Circumstances. For the sake of opening your calendar, I'd suggest encouraging the guest to cancel sooner rather than later. If they agree to do so, all is fine.
One complication, unfortunately, is that the guests' airline may not be as flexible with cancellations, so unless they have a CFAR benefit on their travel insurance, they might feel stuck waiting for confirmation that they won't be allowed to enter the EU before cancelling their accommodation. The entry restrictions are scheduled to be updated every 2 weeks, and member states can make their own exemptions, so it's possible that they won't have any certainty that they're banned from entering your country until far too close for their travel dates for you to re-book them. Realistically, though, the current trends with the pandemic spread make it very likely that the ban on travelers from the US will be extended through August.