Hi Ash: Interesting case, I was just discussing this today, and this issue will come up for debate at the 2017 Summit.
There's not an exact rule and Player A has put himself at risk by inattentiveness so for the moment you have to go to Rule 1. There are reasons for killing A's hand, and reasons for preserving it. My personal instinct is to preserve it for these reasons:
1: A is not exactly "folding" here... he's discarded "thinking he has won" to cite language from Rule 13-B.
2: Rule 53 "Non-Standard Folds" does specifically address folds only... i.e. discarding to voluntarily kill your own hand, which A didn't really do here.
3: Rule 53 also implies intent to fold and forfeit any claim to the pot which you certainly don't have.
4: In the large scope of things we have a bet and call, the bettor just wasn't aware of the call and his cards are identifiable so in the interest of the bet and call IMO the cards should be preserved.
5: Let's go back to one of the most notorious poker incidents of the past generation: Koroknai vs. Baumann at the 2012 WSOP. In that hand the cards were not preserved but rather insta-mucked. 100% that if the dealer had managed to stop the cards from entering the muck they would have been preserved, returned to Koroknai, and action would have been on Baumann... so you have a specific case on which to base a ruling of preserving the cards here.
Note: The TDA discussed a "Baumann" rule at the 2013 Summit at the Venetian but no consensus emerged. I just don't think anyone would have declared his cards dead if they could have been preserved.
Thanks for the great case and stay tuned for the discussion at the Summit.