The cooling system was probably burping. Depending on how he refilled the system, there are air pockets in the coolant. They make the sensors read like you are overheating and everything overflows for a minute while it pushes the air out. A way I try to avoid this is by refilling the cooling system utilizing the thermostat housing. By filling it up from there, it helps get the engine loaded with coolant first, instead of gravity feeding from the radiator. I still have to top the radiator off, and yes, there's a little air from the upper radiator hose that get's stuck. To help that though, I squeeze the hose after install to move air out and suck coolant in.
See if it continues to do this or if it goes away. One of my trucks burped three times prior to working correctly.
1985 1 ton K-5, Detroit diesel, 36" tires (going bigger when money permits), 4-speed manual swap, Rockwell T-case, Hybrid lift, 12.5K winch.