Since the water pump was changed so recently, the mechanic probably caused this. If he was not familiar with the car, he probably did not know about the two bleed locations, one hidden under a clamp in the body intake plenum, for replacing the coolant and the car got an airlock in the cooling system. This can trash an engine, a good reason to only use a specialist for repairs. I have seen many a Mini Cooper destroyed by airlocks.
The overheat may have caused some damage that has resulted in a coolant leak at the pump gasket or seal. I would be back to him to point this out and get a feel for whterh he is really capable of making the car right. You're lucky if this didn't blow a gasket or warp a head.
2004 A6 Quattro 3.0 2005 Jaguar XJ8 4.2
2006 Jaguar XKR 4.2 2004 Jeep Wrangler
2009 Kawasaki Concours 1400