Resolving spells that reveal hand and notes
There aren't really rules about how long you have to take notes, just an implied principle that you don't want it to become Slow Play.
If Anton said “wait, I'm not done yet” and wanted to take a few quick notes, I'd allow that - even though putting the Slaughter Games in the graveyard is the final step in resolution, that's not consistent with how we handle things like Harrow - basically, the Out of Order Sequencing philosophy can serve as our guide, here.
If Anton just reaches over and flips over Nadine's hand “because I wasn't done yet”, then I'd encourage him not to repeat that behavior; either ask nicely, or get a judge involved.
Now, once Anton indicates he's done, then Nadine has no responsibility to help him remember what he just saw; Anton can choose to look at the hand, say “OK, done”, then start taking notes … and if he forgets a card then, too bad for him. The hidden zone is no longer revealed, so it's now Private Information.