Thermostat problem

I took my car for themostat replacement mid August 2024. Yesterday my car engine light came on, I then requested a mechanic to run a diagnostic on my car and I was shocked to learn that the fault was with the themostat. I called the owner who could not help because he said he was in Mosselbay and he said i must drive my car like that and monitor the car temperature. I informed him that i’m scared of driving the car like that and he said there was nothing he can do to assist until the 6th of January. Today is the 30th December, I will be without a car up until the 6th January because of second hand parts that I suspect was used on my car.