Thats a tough one. Typically in a legal sense the one with the qualifications is the one with the most authority regardless of their actual capabilities. That is in my country however. You can't tell anyone about anything without having some kind of permit/licence/or certificate. However, if the dispute was about mass manufacturing in a way that the fitter was well versed in and had years experience over the gunsmith then things may change, but if it were a dispute of old parts, old guns, and generally forgotten skills and tools the gunsmith SHOULD have the more authority.