Shuffling an opponents deck after a Sycamore.

[Phadrex]

Wondering the judge ruling on this. Or ruling in general.

I play Sycamore, draw 7, then while I'm looking at my hand my opponent takes my deck and shuffles then cuts it.


What's the correct ruling and action that a judge would take?

 

[PokePop]

While we try to stay away from Judge rulings in the Ask the Rules Team forum, this one is pretty clear as it is given as an example in the Penalty Guidelines in the Game Play Error section.

7.1.2. Major
When game state has become irreversibly confused due to game-play errors, it is appropriate for the judge to issue a higher-level penalty. Major game-play error penalties are also appropriate for minor game-play errors that have left the game too confused to reset.
In addition to the assigned penalty to the offending player, a Caution should be issued to the player’s opponent for not properly keeping track of game state and rules.
Examples of Game-Play Error: Major include:
* Shuffling your deck mid-game without the use of a card effect.

While it says "your deck" instead of your opponent's deck, it's basically the same.
The recommended penalty for this error is:

Tier 1: Warning
Tier 2: Prize Card