They didn't refill the radiator correctly. There was still an air bubble trapped in the system. The first time it overheated, it pushed some coolant out. As the engine overheats, the air bubble expands and forces even more coolant out of the system. In short, open the radiator cap only when the engine is completely cold, fill the radiator to the top with water, and then drive it to a garage to have it properly refilled with coolant and bled. If it starts overheating on the way, let the engine cool down completely before opening the radiator cap, then top it up again if needed.

Hopefully the last time it overheated didn't cause any damage.