There is absolutely no rule that states the opponents should inform the other player he is about to commit a foul. Would I be a bit miffed if someone did not let me know in some charity event or a $10 local tournament? Yep.
Would I be upset if this happened at a tour event with good players playing for hundreds or thousands? Nope.
It's not up to the opponent to save you from your mistakes.
With no refs in most pool tournaments, the players are the refs. Your example does not really match the question though, first case was a player that fouled saying they fouled, in the example it was the opponent not pointing out the foul before it happened to stop the player from fouling.
I fully agree, as I am sure everyone else would, that if you know you fouled, and there is no ref, or even if there was, it is on your own sense of honor to let the opponent know you fouled. I have seen plenty of pro events where the ref made one call, but the player thought he committed a foul and basically over-ruled the ref. In the weekly tournaments I play, I have seen players walk away to get a drink or go to the bathroom and if the other player fouled while they were gone, they would come back to ball in hand waiting for them.
This actually happened to me a week ago during a doubles tournament, I made a very close hit, which seemed like it may have been bad. So I said "I think that was bad" since I was not really sure and did not want to call it a good shot. My opponent said "no, it was a good hit I was watching it". So we both did the right thing LOL. I was going to accept the foul since it may have been, the opponent said I made a good hit even though I was going to give them ball in hand. That is how I like the game played.
