Slippery sleeves corner case
Instead of giving an answer that's a direct answer to your question, I'll be giving a more general response on how to deal with corner cases themselves, in the context of this scenario (as this should be far more helpful for other situations). So, let's look at a certain clause in the IPG: “The Head Judge may not deviate from this guide’s procedures except in significant and exceptional circumstances or a situation that has no applicable philosophy for guidance.”
When dealing with any apparent corner case, we first need to determine if there is, indeed, any philosophy we can use to guide us. The IPG doesn't cover every scenario, but it covers all the likely and common ones. Looking through the IPG, the closest we really get to is Looking at Extra Cards, Hidden Card Error, or a Game Rule Violation. Specifically, this portion of the LEC philosophy is important: “A player can accidentally look at extra cards easily and this infraction handles situations where a dexterity or rules error has led to a player seeing cards they shouldn’t have. Once those cards have joined another set, the infraction is handled as a Hidden Card Error or Game Rule Violation.”
Now, this is where things start to get a little tricky. Technically, by the letter of the IPG, if those cards from the library joined Abner's hand when they spilled over his hand, then this stops being a LEC issue, and is now a HCE/GRV issue. At this point, the cards that fell from the library are, more or less, part of the player's hand now, as they have been mixed. Between the two, the closest match is HCE - the GRV remedies make no sense, and the HCE is “closest” but not a perfect match, as there are still disqualifiers for this situation. Looking again at the philosophy and remedies for HCE, it's evident that they were written with the intent of a player making an error on their own zones, not the opponent doing such.
At this point, we now know that there's not really any directly applicable guidance. As such, a deviation should be used, while still utilizing the philosophy as best we can. Check if both players ever knew any explicit cards in the affected hand. It would be fine to ask the affected player what cards he remembers in his hand, replacing those IF they're in the jumble and you're confident that the player is truthful (they really don't have an incentive to lie, depending on how much of the library slid into their hand), and replacing the completely unknown cards by random cards from the remaining set of the hand/library mixture.
But, even with that deviation, what are the weaknesses/strengths? Does it try to do too much, giving one player an unfair advantage? How long would it take you to come to such a deviation and apply it in a tournament setting? These are the questions you should be asking yourself in corner cases like these when they arise (rarely). Hopefully this general guideline is helpful.