For a more advanced understanding, if you listen as Mark suggests to Charlie Waller of the Country Gentleman or Bill Harrell with Don Reno and elsewhere:

Count and pat your foot in order to 'feel' whether they are playing directly on top of the beat, or whether, for the purposes of the song, they have chosen to play ahead or behind the beat on the two and four (assuming 4/4 time).

The answer to your folkie player is, Les B., "It just don't work that way."

chance