I read the post and every comment and no where do I mention DTV (I am aware that it is a DTV group and that the OP asked a non DTV related question).
I tried to correct you. Yes he is coming as a tourist. Yes he will get an entry stamp that allow him to stay 60 days. No he will NOT enter on a tourist visa!!! That is what I am trying to tell you. He will be allowed to enter without any visa - visa exemption.
A tourist visa is something you apply and pay for BEFORE you travel (it is issued by a Thai embassy or consulate).
his passport will not be stamped with a 60 day tourist visa - he will be allowed to enter WITHOUT a visa (visa exemption) and can stay 60 days etc. A tourist visa is a different thing.
The local Immigration offices have their own requierments, hence it can vary...
The link you added was to the Immigration Office in Nonthaburi. If that is the Immigration Office where you will do the extension, I would rely on that information.
maybe no one answer the question because they read the requierments and actually understood the meaning of this: "It is important to note that you are only able to apply if you are currently not in Thailand..."
Also, one requierment is "document indicating current location". This can NOT be Thailand.
Do you keep questioning what everyone has commented so far, because you don't like the fact that applications should be made from outside Thailand?! 🤔