Tenant just moved in, and now needs to move out

I swear this type of stuff might only happen to me. After a lengthy screening process on my BRRRR, I signed a lease with my tenant on November 1. Everything is good, received deposit and first month's rent. Just received a message from them today that they cannot move-in and need to move to another state due to a family member getting suddenly ill.

My radar is up (it's always up) but I want to understand the right process here. I'm thinking as long as they haven't damaged anything, I can do a deposit return/key return with them. They paid for a months rent, which I believe I should hold onto unless I can move someone in ahead of time, and then do partial rent up to that point.

Any thoughts/suggestions on proceeding? Anything I am missing? Anything I should do/shouldn't do?