Although it makes sense, id agree with the others here and wouldnt trust a Kmart Mechanic with installing a windscreen wiper blade...
When i had my other car (1979 Mazda 323) i took it to them for a disk brake machining. After 120 bucks later, and them telling me it was all good, my mechanic mate had a look at their work, they didnt do anything! We took it back to them, they said it was a mistake, they THOUGHT they machined it, but theyll do it there and then at no charge. After 5 mins, they eventually found out that the disk diameter was too small for their machine!! They eventually gave us our money back.
But enough ranting...
If it is just a thermostat problem, buy another one with a smaller range, and test that. Thermostats only cost a few bucks anyways... alot cheaper than going to a mechanic and being told they cant fix it...
Why he couldnt test that... i dunno...