Before reading other responses:
This is indeed a tricky one. I would want to first investigate whether Abel had any knowledge of the top card of his deck, possibly from a previous turn's scry. If he did have that knowledge, it's important to check whether this is an honest ‘oops’ or an attempt at not taking high cmc damage. I'm assuming for the rest of this answer that we've determined it was an honest mistake.
Pain Seer triggers during the untap step, and has a non-optional, though not usually detrimental, trigger. Abel forgot this trigger, and took the turn-based action of drawing a card. This in and of itself would just be a GRV with no warning. Had a judge been called at this point, the opponent would have had the choice to place the trigger on the stack or not. The opponent would almost certainly choose not.
However, upon realizing that the trigger was forgotten, instead of calling a judge, Abel immediately resolved this trigger themselves, drawing a card. This is a pretty big problem, and lands us in Drawing Extra Cards land.
At this point, I'm concerned over whether this is Drawing Extra Cards, with a game loss, or whether a previous Game Play Error has previously occurred (namely, the entire resolution of the trigger in the first place, including the reveal of the swamp).
On re-reading the IPG, though, we find
If the identity of the card was known to all players before being placed into the hand, and the card can be returned to the correct zone with minimal disruption, do so and downgrade the penalty to a Warning
. Given that this is either a GRV-warning for resolving the trigger early, or a DEC-warning, returning the swamp to the top of the deck, I think the difference is mostly academic.
I would call it a DEC-warning for tracking purposes. Return the swamp to the top of the deck, and then shuffle the unknown portion of the library (which includes the swamp). Remind Abel that when they realize they have broken a rule, they should call a judge instead of trying to fix it themselves, and impress upon them that this was nearly a Game Loss.
now, after reading all the responses:
Other Judges point out that Nancy immediately called a judge, and so does not earn a FtMTGS. This is true, I just didn't think to add it.
There is quite a bit of debate over whether this is DEC, L@EC, or some other infraction, and what the correct fix is. In particular, the first illegal thing that happened was that Abel revealed an extra card (L@EC). Then Abel drew that card. The IPG states that after the card is drawn, the offense is no longer L@EC. On the other hand, if a GRV precedes drawing extra cards, DEC doesn't happen. It seems like there's a bit of circular logic here! Given that L@EC's exception is the more specific of the two, it makes sense that a L@EC followed by a DEC should apply L@EC's exception, and not DEC's. That said, given the ambiguity, we should take a moment to consider the philosophy.
Looking at Extra Cards accounts for errors in dexterity, where cards are accidentally seen. It also accounts for game rule violations that give a player extra information. Also, while not given as an example, it accounts for situations where a player begins to draw a card illegally, does not do so, but has seen the card. In all 3 situations, the player has extra information, so we need the more specific L@EC to authorize a shuffle as part of a fix, instead of a general-purpose GRV. I'm not entirely sure of the purpose of the exception - if a card is actually put into hand instead of just looked at, then the card was ‘drawn’, and DEC applies.
Drawing Extra Cards accounts for any of the number of ways extra cards could end up in hand. It exists as a separate rule because the potential for abuse is much higher. The exceptions account for situations where there's less of a potential for abuse, as the card has already been seen. The other exception, I believe, accounts for the situation where an error makes a player believe they can draw a card when they cannot, and the opponent would be tempted to silently allow the GRV to catch the player in a more severe DEC.
In this case, while the sequence of actions was too fast for Nancy to interrupt, I feel that there was a specific infraction - L@EC, that preceded and was linked to the next problem - DEC. Had game play been a bit slower, and Nancy had a chance to Maintain Game State, then either the error would have been caught at the reveal and the extra cards never drawn. Or alternatively, Nancy would have failed to maintain game state (unintentionally or intentionally), and the card would have been drawn. I think the same penalty would apply to Abel here - L@EC.
Some judges asked why we would apply the fix for Missed Trigger when this was either L@EC or DEC. Even though we only apply the penalty for the most severe infraction that occurred, GRV-MT still happened, and the opponent should get the choice to force the trigger or not. By the same token, an extra card was looked at, so L@EC still happened, and we should apply the fix for that, so we need to shuffle away the swamp.
So in conclusion - after reading other responses, I've changed my mind. Abel gets a GRV-L@EC with a warning, instead of a GRV-DEC downgraded to a warning. Still no FtMGS for Nancy. Return the Swamp to the library, randomize the unknown portions of the library (including the Swamp), and then offer Nancy the choice of whether the trigger resolves.