HYUNDAI - Hyundai Grand Diesal engine dismental in just km

let me showcase the report , I visited (*rd sep **** ) to prestige service centre , entered with my car at prestige premises , greeted well by Mr Aditya Nayak and He aligned one supervisor Mr Mohan to check my car , he checked and said that engine oil  bit less as service is due and was there to get it serviced , my car issue is that black smoke coming out from the silencer and i asked to check the same , he just opened the top lid of the radiator or etc from which white smoke was coming out. 

than he took me to his reception desk and said that sir " Isme toh engine ka kaam hona hai and it will cost *.** to *.* lacs" , i was shocked and asked can you please elaborate further- he said pistons, rings has to be changed , i asked how can you say the same without checking the car ,his simple answer was that "experience"

Then i asked him to write the same on performa that my car has this issue and will cost around *.** to *.* lacs and he denied to give in writing. 

I mean what is going on here , without checking you are giving me an estimate and saying it's out of warranty  and the last service is not per HMIL , but how can an engine breakdown @*****km . how?