Moving this to the Judge's Chambers because this is not really a Game ruling, it is a Tournament ruling.
I would treat it as equivalent to looking at the opponent's deck and/or Prize cards.
Unless there are mitigating factors, this is pretty serious. Evidence of it not being on purpose or sheer ignorance would be the burden of the offending player.
If done on purpose, this is Cheating.
There is one and only one Penalty for Cheating, regardless of how major or minor it is.
That Penalty is Disqualification.
I'd have to be convinced that they player didn't know what they were doing.
I would not be easy to convince.
Even if I were convinced, the penalty would have to be serious, ranging from Prize Loss to Game Loss, to impress upon the player that this action is not to be repeated.