Just to add closure here ....
We notified the guest that first night (Sat) that we would be there the next day by noon with heaters and to look at the heat system. They responded the next morning (Sun) and reported that they had a very cold uncomfortable night and had decided to cut their trip short. The would spend time in the city while we were in the condo, then leave that afternoon. They had been booked through to Monday.
We arrived at noon. They were out. The place was chilly and the heat was not working. We warmed it up with space heaters and after about an hour my husband got the heat system going. We left at about 1:30 and updated them. They said they had already cancelled their evening plans and were going to collect their things and drive home.
They officially checked out at 2:30 and once I confirmed that they had left the keys in the lockbox, I refunded their whole stay.
Throughout all of this, we were extremely apologetic, kept them apprised of exactly what we were doing, and assured them that we'd refund their stay as appropriate. They were extremely understanding, kept thanking us for trying to make it right, and from what I could tell, were more disappointed than angry. Though guess their rating will tell the tale ....
Thanks everyone for your quick response here. All good advice!
-K~