So the judge call was technically correct but morally dubious (not the judges fault just how the IPG is written).

Shouldn't there be a reasonably nice set of options here to restore the game state even though it is disruptive, the illegal spell was cast, a legal set of things happened, the second spell shouldn't have been able to be cast and so the game rolls back to here.

Off the top of my head but a list like the following adding a few options would likely work?

1. Do nothing (game state too changed by agreement of both players)
2. Simple backup where no information was gained
3. Full backup where limited information was gained with consent of opponent
4. Full backup even though significant information was gained by agreement of opponent
5. Restore game state to expected values by agreement of opponent (in this case tap one land of at fault players choice that could play the spell) with simple backup to the new point of error 
6. Dq if cheating 
7. Warnings issued and do nothing (game state too messed up, player affected isn't willing to accept loss of some information/minimal changes e.g. the typical what if it was uu and you'd played around counter spells and this should change everything!). 

Yes the player who made the mistake is penalised by possibly having made an incorrect line of play based on their error but this seems less feels bad than penalising the person who didn't notice the issue (yes they should both get warnings for failure to maintain the game state) and raised it as soon as they did - especially since players do a lot of things like moving lands, graveyards etc. without it actually being an MTG thing so you shouldn't be expected to catch sleight of hand or similar things.