"A caveat against using a visa agent.". He did not use a visa agent, he uses someone to forge fake stamps while not being in the system. When you use an agent you are inside the system (computer), legally, like everyone else.
Agent allows you to be hassle-free. I know the 60 days extension is 1.9k baht but it is location dependent. For a little bit more you can get it without any worry. You can also get other type of extension like medical, or visa like Non-O or Volunteer based on Non-O.