Hello @Alice1388
I presume when you checked the listing it does have hot water?
In that case I would message the guest and say you are sorry to hear they had issues with hot water but surprised that if this had been an issue since their check in that they didn't say anything until they were due to check out, thus not giving you an opportunity to try and fix any issue.
You can also confirm that you also live on the premises and on checking the listing when they left there was no issues with the hot water.
Do make sure you leave an honest review to warn future hosts and also to put any review they leave in context.
Absolute no to a refund.