My husband's terrible experience with repair

My husband took his beloved i10 car to Hyundai Wonderboom as it was not starting properly. In Sept 2017. It went through a diagnostics test and the problem was resolved even though the report stated there was still rust in the coolant system. This week the car had problems again and it appears the gasket had blown. It appears due to a misdiagnosis in Sept. Now my husband was presented with a massive quotation to fix his beloved car. This is so disappointing of Hyundai service centre . I hope that they will fix his car.