I prefer the second option as a default. This is a really terrible situation where even if you allow the last bet to be taken back, the player has already lost the chance to get the other player to fold -- he may not even want a showdown so he may be screwed anyway.
However, at the end of the day, I think you have to stick to the letter of the law here - A has to protect his or her own hand, and needs to be sure that his bet is in fact called before turning cards over - even in the face of dealer misinformation.
That being said, I wouldn't protest to applying the first option if the amount to be returned is significant.
I cannot remember where I saw the clip, but there is a video out there where the announcer (l think it may have been Linda J) said something that made the player reveal his hand prematurely - that was an interesting situation showing that the misinformation need not only come from a dealer! I would like to find that clip....