Start the car and let it idle. Spray water And dish soap mixture ska soapy water on e very flange and connection on the exhaust system . Especially around where the work was done . The fact that the guy put putty o. The exhaust verses re weldind the flange screams shoddy work . So check for leaks as mentioned and if you find some have them properly welded up . It’s entirety possible that the HVAC system is picking up exhaust fumes from a bad exhaust leak and blowing them in the cab . Let me know if this works . I’ll help u further if not