When a 2 family house has only one thermostat(one zone) and the thermostat is located in the downstairs apartment next to the fireplace,when the fireplace is on will the boiller start.is the upstair apartment will have heat?

Under normal circumstances it is not wise to put a thermostat near another heat source like a fireplace since the operation of the fireplace and the heat generated by same will have an negative effect on the boilers ability to heat the rest of the building. Some idiot must have done the thermostat install or a landlord on the first floor who does not care if the upstairs gets heated.