Well, I suppose we can't definitively prove what Aral knew from what he says to Cordelia, but assuming he's not a bald liar or a sociopath--he fought duels with both of these men, which involved talking to both of them, and he does not report that either of them denied having had affairs with his wife, or voice any uncertainty on that score. He regrets killing them, and he realizes that killing people to recover his honor on that score was futile, but he doesn't express any further uncertainty over the justice of what he did.