@Rebecca735 Of course a guest, whether he is staying at full price, a discount, or for free, needs to be held to your house rules and expected to pay for any damages. And yes, charge a cleaning fee, as well as a security deposit (although Airbnb doesn't actually put a hold on security deposit amount, it may serve as a deterent to carelessness). In fact, I don't see why you shouldn't charge enough to cover your costs- utilities, amenities, etc. Free accomodation and not caring to make a profit is one thing, but I don't see why hosts should be out-of-pocket.
But have you actually checked out the credentials of this person who is inquiring? Hosts have been reporting a lot of scams of people wanting to stay and claiming they are a doctor or nurse, but are just trying to scam a free stay. Be really careful and do your due diligence. Did you sign up to Open Homes, or to host COVID responders?