I don't think the answer is covered by the comp rules or the IPG.

In that blog post there's a ruling about what to do if someone misses their Dark Confidant trigger, then the opponent draws, and then notices the missed trigger. I believe this is an identical situation! Do you all agree?

I just noticed it's been 31 days since anyone chatted in here so I think I'll go ahead and post an answer.

@murgatroid99 Just the person I was hoping would appear

Foiled by the question author! I had a feeling I was going to post something incorrect.

I think one way to look at it is what the ideal situation would be, and how the current situation diverges from that and how to get closer to the ideal. If player B had 2 cards in hand then Expert entered, then one of those two cards should be gone. But after B's draw step, there are now 3 possible cards to discard. The card they drew could be better or worse than their current 2 cards, and there's no way to know other than "trust me, this was the card I drew, these were the cards before"

If player A doesn't trust that, I think the closest you can get to ideal would be a random discard. Player B isn't able to use the additional information to benefit themselves, and their hand quality is now random.

As for the dark confidant situation, the biggest question is: did the drawn card allow them to survive the the trigger? Again, if trust can't be established, then I think the closest to "Amy only has X cards to deal with this instead of X+1" would be a random shuffle into the library, then dark confidant, then regular draw (if survives)".

One additional thing about resolving it these ways would be the gameplay warnings. Ok, we tried to make it as close to ideal as possible, which likely benefited one player or another, but don't make a habit of it.