regardless of who it is (and seems I was off with my guess so appologies to TDI) I think it is rude to do a song that somebody has just done without at least some acknowledgement that you have noticed. If it was a festival folk club session then surely any material is fair game for anybody - why should a floor singer know whats in the repertoire of a booked act? Unless it was written by them, which sounds like not.

If the booked act had noticed they then had the choice to as suggested above say that they're going to do another version or to swap something else instead.

If they hadn't noticed then they weren't paying attention, which *is* rude and *would* warrant an apology.