"It's hard to listen to someone who can't carry a note with a co signer"

I think you're going for the low-hanging fruit there, kendall! Question is, when (if ever) would you think it appropriate to go up to somebody who sings reasonably well and say "shame about the way you screwed up that second chorus"? Assume that they haven't shown any sign of thinking that they did screw up the second chorus.

Actually an even more interesting question, from my perspective, is how you'd feel if somebody else sprung this on you - assuming, again, that you'd thought the second chorus was fine.