Harbor Freight and buy a cheep "Thermal Gun" and actually check the engine 5 minutes after it's started and again when the alarm goes off to know for sure. He can also get a cheap magnet style thermometer that sticks to the engine and test it that way. You need to know for sure what the temperature is. He doesn't have a cooling loop but he does have a thermostat which could be sticking or as was stated, an injection elbow clogged. Once you eliminate a cooling issue it will take the mechanical out of the picture and only leave the electrical. Someone mentioned a bad ground. I would start there. The system is pretty simple with only a few connections but I would suspect that it might be a bad sensor. As the engine heats up, one side of a bi-metal strip in the sensor heats up faster then the other side. The result is that it bends the contact in the sensor towards the sensor post connecting the post to ground. The other end of the sensor wire which is connected to the post is connected to the grounded side of the light or buzzer. The hot side being energized when the key to the engine is turned on. It may be energized through a pressure switch which is energized when when the oil pressure comes up after the engine is started. Either way, the system is simple and easy to check but it might be worth it just to proactively change the sensor. They're pretty inexpensive. Good luck, keep us posted.