95 Odyssey Temp Gauge

I'm working on my sisters 95 Odyssey. What she has eplained to me is her temp gauge is dropping when she drives on the highway. Her mechanic friend says its the thermostat that is the problem and serious if not changed. I have had the temp over heat but never under heat so in other word dont know whats the problem. Also the heat in the car is semi warm but not blowing. I've read it could be the blower fan.