[EnderW27]

We're playing 3-6 with a kill. It's recently been on a kill but for this hand it reverted back to 3-6.

On the river it's heads-up and I'm out of the pot.
First to act bets $12 and the second guy immediately folds.
Now, based upon the board and his betting pattern in the hand I put him on a draw and that draw didn't get there. So any bet probably would have gone unanswered.
But $12 was the incorrect bet and no one corrected it. For all I know I was the only one to notice it.

It all happened so fast and the only reason I didn't really call attention to it is because, as I said, I figured the guy was folding his busted draw regardless of the bet. But do I have any obligation, ethically, legally, otherwise to point out this error?
I can't imagine that I do and yet...someone's gotta stand up for proper procedure, right?