Marion, don't stop him and make him start over in your key; that's too emarrasing. The thing to do is gently set your guitar down and gracefully stroll (glide is really the idea) over to his side, raise your voice a 5th above his---in his key---and complete the song together. You'll both look good and sound good too. The faithful will think you just worked it out that way. Oh, it helps if you fix your sincerest gaze on the loftiest stained glass window in your field of vision.

- Mark