She should do the talking to Airbnb, @Christina1250 . If you wish to refund her, you can put that statement in the messaging. I have done that for guests when I felt their situation merited it, and it turns out fine. The guest calls in, then you get a call from CS asking whether you are still good with it, and you say yes.
If you choose not to refund, that's fine, too. We have cancellation policies for a reason. The guest had the option of getting travel insurance.
@Christina1250 The guest needs to call Airbnb, say she has Covid and they will cancel and refund her in full. Covid comes under the extenuating circumstances policy that overrides a host's cancellation policy.
I wouldn't voluntarily refund the guest. If she has Covid then she needs to apply to Airbnb for a refund via their extenuating circumstances for Covid and provide them with proof that she has it.
There is no need for you to do anything. Apart from you might want to familiarise yourself with their policy around when guests can cancel under extenuating circumstances and advise your guest to contact Airbnb.