Conrad, again you are confusing two different types of performance.

If a group of friends get together to sing or play tunes, whether that's at home, around a campfire, or in a pub, then quality may be secondary. It is primarily a social gathering. That's the sort of situation where someone like Don's friend might perform without embarrassment. Anyone overhearing such a 'performance' will recognise it for what it is, and won't draw any conclusions about folk music as a genre from it.

It is very different when you begin to organise public events to which you expect to attract members of the public. Then you have a responsibility, firstly to the audience who have taken the trouble to come along, and secondly to the music itself. In this situation then it is not unreasonable to expect a certain level of competence in performance, and an understanding of both the genre and of the individual songs. This is particularly true if the audience is paying to hear the music, but also applies even if the event is free. If you present them with performers who are rubbish, not only will they not return to future events but they will come away with a very negative impression of folk music as a genre.