Seems to me that you are jumping to conclusions not in evidence. I read nothing here to assume shoddy work. Possibly this individual is not capable of doing the job but, he was only questioning sizes. Even if I was not capable of doing the job I would want enough info to be sure the job was done properly. I have seen many gas lines installed by professionals and gas companies which were under sized. On the other side, your advice is good in that if individual is not experienced he should hire a qualified person.

: Hey, I have a pain in my left side. Maybe my appendix or liver. Cut me open and take a look....
: What, you don't know how and it might hurt me...

: Well guess what, poor gas work kills people all the time. If you have to ask a basic question such as this, put down the wrenches and hire a qualified person to do it. Just like you do for pains in the side.