as derivative from that, your advice is invalid: the information of A's experience of X is critical for B to understand that it did anything wrong in the first place. A has to give B that information.
once you have a full picture of the shared platform of interaction - B's reasons for X and A's experience of X, only then it can be said that B might have to "understand on their own" why they did X, what elapses of judgement they might have had, because there can certainly be information about it that only they would have.
but ofcourse, that's assuming that what appears to be the full picture is the full picture, and given natural human limitations, it rarely is, which is why a back and forth exchange of information is valuable.
but let's say that you do have the full picture. even then the statement of "find out on their own" it isn't quite true, since in the end of the day we aren't all special snowflakes, and because it is in the end of the day a rational problem happening to two subjects within a shared reality within the larger subject field in which everyone has gained some expertise in - the art of being human - and as such, any extra perspective and experience can help B gain insight into the elapses in judgement. while it's true that A might be biased in that regard, it is also the most likely to have invested interest, and as such, still useful.
on a side note - another derivative is that to maintain that, A has to be open for that information, and it would be impossible for A to judge their actions truthfully without it.