I totally subscribe to the whole "do unto others..." thing, which is exactly why I don't tell my opponent. If I shoot the wrong ball in a match, then I deserve to give up bih, and it's not going to make me feel very good about winning if I only do so because my opponent tells me I'm getting ready to screw up. I'd rather lose the game than have my opponent save me from what would have been colossal mental meltdown.
To me, there is no question of ethics involved in this situation. It's a very straightforward case of knowing that your opponent is getting ready to make a mistake, and it happens frequently in many games. It's no different from any other time when you're watching your opponent prepare to shoot a shot that you know is not going to turn out good for him; maybe he's trying to trying squeeze whitey through a space that, from your perspective, is clearly too small, or maybe he's less experienced at bank shots and he's getting ready to fire at a bank that you know has a dead kiss in it, or what if he's getting ready to juice the cb, but from your seat you can clearly see a big shiny spot on his tip, which tells you that he's in imminent danger of miscuing. Do you warn him in those cases as well? If so, then either he should be paying you for lessons, or his parents should be paying you for babysitting.
Obviously, it depends heavily on the game. If we're talking $5 nine-ball amongst friends or a league match or something, then yeah, I'll probably tell him. But in a tournament or money match, he's on his own; that's why pool is called an "individual" sport.
This also reminds me of a situation I heard of recently in a Golf tournament. Mark Wilson (not the pool player) had just hit his shot, and one of the other players in his group was talking with his caddy about which club to use. The other player's caddy stated that Mark had hit x club, so maybe he should hit y club. Mark's caddy, overhearing the conversation, interjected, stating that Mark's x club was a different degree than standard, and that they should adjust their club-selection accordingly. Mark, being a true sportsman, called a foul on himself, because it is apparently illegal for one golfer to advise another in a golf tournament. He suffered a two-stroke penalty, and his caddy was literally in tears after the round was over. Happily, he went on to win the tournament. If you, like myself, consider golf to be the ultimate gentleman's sport, then you should find this scenario to be quite interesting. I view the "opp. shooting the wrong ball" scenario exactly the same way. Is it fair to the other participants in the tournament if you help your opponent beat you? I think not.
As always, JMHO
Aaron