Overheating car: two sets of symptoms depending on thermostat?

I'll try. I presume 3.8l 6cyl. First did you install thermostat right way up, spring part should be down in intake. If coolant not circulating in system rad. fans won't come on, so turning heater on acts like radiator. gauge won't register overheating if coolant not circulating. Try leaving therm. out and see if everything works OK. The cap blew off from too much pressure which also caused rad. to blow. Have seen therm. installed wrong way many times in my career. If that's not the problem before reinstalling therm. see if coolant is circulating in rad. run with rad. cap off should see coolant flowing. If not could be bad water pump impeller, but if you see air bubbles in coolant could be bad head gasket. Got to be one of these but strongest possibility is thermostat upside down. Not doubting your mate's prowess but things happen. If more input needed email me.