A composer has absolutely no say in how a tune is played in a live environment (and, in the case of folkie stuff, would probably never know). It's called "interpretation" and, no matter how much the composer might object to it, and however naff it might sound, the performer has the right to do what he or she wants with the tune. Artistic license and all that. There may well be some legal block to recording it without the composer's permission, but I wonder whether happens in practice.