@Billie37 It does sound odd. I wonder if they used a lot of hot water for dishes or laundry just before trying the showers, so they had used up all the hot.
And OMG, they let the water just run for 10 minutes? What dummies, like it would suddenly get hot after 10 minutes- if it was a matter of them having used the whole tank for dishes, or whatever, that would just make it take longer to heat up again.
The messages don't really sound like they were scamming for a discount, but who knows.
But here's something that sounds odd- is this guy actually staying there, or is this a third party booking? He says "I reached out to my family..."
I don't know if it's at all pertinent, but I stay with a friend of mine sometimes who has one of those single handle shower faucets and several times I couldn't get any hot water out of it. When I told her, she said her sister had experienced the same when staying once. Other times it was fine, it seemed like an intermittant problem without any explanation. But I seem to recall we googled it and it had something to do with the hot/cold mixer in the faucet being faulty.