Fix the leak first. Use a pressure tester or you can add dye, then search with a black light. Once there is no air in the system, watch how high the temperature gauge reads. If it is lower than normal, suspect the thermostat. If it reads in the normal range, feel the two heater hoses after the engine is warmed up. They should be too hot to hold onto for very long. If they are cool, suspect a plugged heater core. They can usually be flushed out with a garden hose but be aware that sometimes the acids that build up in the coolant will corrode a hole in the heater core that is blocked by the trapped sediment. It could leak after the flush. It is common for people to blame the mechanic because the leak becomes evident right after he flushed the system.