Why did he not just go ahead and replace the thermostat? They are cheap.

Running with the thermostat is a VERY bad idea in ANY climate. It's probably worse in a cooler climate.

There are two reasons for this:

First, the coolant will never reach a temperature that will make the heater very effective if you need it.

More importantly, your oil temperature will not reach the level that it needs to be in order to boil off the moisture in the oil. This will cause the engine to sludge unless you are changing the oil every few hundred miles.

Additionally, if the only way for the engine to not overheat is to block the thermostat open, there is something wrong with the engine or cooling system.

Good luck,