Mark-
This business of changing a minor chord into a major one by putting in a major third, usually at the very end, or at the end of a section: it's known as the Tierce de Picardie (Google will supply more info if required, and it may well have been explored in the Forum). I don't care for it much myself in a folk context, but am resigned to the fact that some people will do it.

The worst thing about it is that there is no consensus on this, so you may get both a major third and a minor third at once, which clashes and is not what anybody wants to hear. If it's a song you're leading, there's not a lot you can do about it, because you'll be singing a note from the melody, generally the tonic note at the end, and you have no control over what harmonies people put in. I suppose you just have to be glad they're taking that much interest in the song.

Marje