@Paul154, it's probable, that the guest is not lying, but feels cold.
1. There is a problem with electronic thermometers: their functionning produces a bit of warmth, more so if they are on wifi, so they are always showing higher temperatures. If the sensor for the heating or the thermometer itself is close to the source of heat, they are fooled. If the radiator is somewhat cofined and has the sensor on the radiator, not on the other side of the room, it's rather far off the mark. My new fancy radiators are only correct with certain temprature levels outside. I start with setting them to 20 /18 C. As it gets colder, they are now at 21 / 18,5 (18,5 being the highest night level). During the winter, that goes on to me manually switching up and down on the day level and if it's really bad, the radiator may believe it's producing 26 C, when the old fashioned thermometers show 20.
The guest may feel cold being sick or not moving a lot or sitting around in a t-shirt in winter. If I come accross that with my inhouse guests, I lecture them a bit, may even plant a malicious seed, that feeling contantly cold is something to be checked with a doctor. And offer a warm jacket to wear 😉
If you rent a full unit, you could put a limit on electricity consumption, especially, if they can check the meter themselves. I give them an average consumption of last year for the period and write in the contract, that they have to pay, what is over 150 % of the average. With a few rules how to keep the consumption down. Since I do that, I did not even have to check more than by a short look on the meter - doors seem to close themselves now, hot baths must be fewer and showers shorter 😉
I do a lot of one night stays in a shared place. The guests may set their ow temperature for the night and adapt it as they wish. I tolerate everything between 18 and 24 C, but get into discussions, when they want to have it colder than 17 C / 62 F and switch off the heating and open the door wide, when it's over 25 / 77. I don't want to take my shower in a very cold bathroom and I can't prepare breakfast suffocating. But it happens a few times a month to have such extremes one after the other.