Even though I'm a million miles away from you across the pond, in the NW corner of America (Seattle area)I'm enjoying this thread greatly. I have two thoughts:

If it was my gig, hoot, jam, house, class, whatever ... sometime during a break,I might get that person to one side and say something like: "I really like that song you sang. How about if we get together next Sunday afternoon and I can teach you some more words." That might give me the opportunity to work with the person on other things, such as pitch, or whatever?

This thread reminds me of a situation that happened many years ago when I was teaching a guitar class at my local commuinity college. I sometimes would let a student or two end the class by performing a song. This one night, the fellow who performed was beyong awful ... he was simply terrible. After his presentation, being the teacher and trying to be postive, I spoke postive things about his song. I don't remember anything that I said, but the next week, I got many private compliments about how I handled it.

People are people ... we're NOT all perfect ... but it's fun when we try ... eh?! ... CHEERS ... bob(deckman)nelson