@Swils Google Nest don't manufacture boilers, do you perhaps mean the owner left the Heat Link unit but took the thermostat itself with them? Does the unit look like the photos linked below?
Learn about Heat Link for Nest thermostats in Europe - Google Nest Help
Assuming that's the case, if you don't want a smart thermostat, you would need to ask a plumber to remove and replace the Google Nest Heat Link with a manual thermostat. In the meantime, assuming you have a combi boiler then your hot water should continue to work on demand and you should be able to press the large button on the Heat Link and use it as an on and off switch to manually turn the heating on and off.
Manually control heating when Heat Link is disconnected - Google Nest Help
