you don't know what actually happened when the safety check was completed , so answering your question is a lot of if/buts and why tbhThat is of course true. It is the reaction to hearing about these faults that varies from abusing the person making the critique ( a few on this forum ) to other RGIs ( not on this forum ) who would take their colleague / apprentice to task if they made the same mistake.
The bottom line is the RGI left the house without having ensured the boiler was in working order when he left the house. Why would he test the boiler was working and then turn the gas off after testing the boiler fired up correctly ? Instead of abusing people try and explain how that could have happened.