Engine Cooling problem
2002 Holden Vectra 4 cyl Front Wheel Drive Automatic 120000 miles

HI, we just bought this car from a private sale. The very next day car got overheated while my husband was on his way back from work. there was team coming out . My husband checked the coolant , which was empty. he bought some coolant and filled it up , and drove the car home stopping every few min. The car was sent to mechanic, who said there was a leakage below the radiator. He repaired the radiator, half the cost was beared by the previous owner. Mechanic did the pressure test and stuff and advised it all good now. the very next day the car started overheating again. it is fine if we driving it in less traffic and/or at above 50-60 but othrewise it starts getting overheated. Mechanic said he filled up the coolant tank with 4 litres of pure coolant but when the car overheated,. my husband checked and it was empty.
Can someone advise what is the issue, is it busted head gasket?