If you can get your hands on something like GM dexcool coolant - then clean out all the cooling system including heater and expansion tank, install coolant and distilled water mixture. it will work for about 100,000 kms without question and will never cause any rusting either.
If you cannot find dexcool - then any modern long life coolant can work too.
this picture shows the internals of a GM truck radiator using dexcool - The finny thing inside is the transmission cooler.
your mechanic is flying by the seat of his pants