If the person is open to it, there is a clear advantage to having a competent instructor, no matter what the delivery method is. Starting from no musical background and getting good is very challenging, unless there is a truly natural talent latent in that person, but even then, how many times have we seen a self-taught master doing silly little mistakes and often even some very serious mistakes (playing with 2-3 of the 5 fingers or playing only while pulling, jamming the accordion closed and playing while pulling again. Unless its a diatonic, whats up with THAT?
), that a person who is taught by an instructor... using any method... wont ever do.