Why did you offer 50% off the entire stay when you send a second supplier out to fix the issue. It sounds all they were entitled to was a small discount for the first night @Elizabeth3082
Surely you would have know from your supplier that went in for the second time as to why there was a continuing issue with the hot water.
The guest can still leave a bad review so please don't let yourself be blackmailed.
Obviously you can't change the goalposts know you've already agreed such a large refund.
Hopefully you left an honest review to warn your fellow hosts.