First, do the refund of the last night.
Then send the guest (via AirBnB messaging) a note letting them know that you were sorry at the issues they experienced with the shower, and you have refunded the final night of their stay as a good faith gesture. You hope that they will consider the steps you have taken to show how serious you think this situation was and how much you are sorry it impacted their vacation.
Don't make any more of it than it is, though. Don't take it too hard either. You are a host. Your property suffered a break-down when you had a guest. You took care of it immediately and didn't try to profit off of it by not understanding that this DID affect the guest. I am really glad you were gonna give them a credit - and that was your first thought. It makes the rest of us look good!