As Ceribia referenced, RAW does not allow this. However, in this case I would consider it for four reasons.
First, the Deck of Many Things is an artifact. Artifacts generally give the big flaming middle finger to RAW. It is what they are there for, to bend or break the rules in epic, awesome, or sometimes silly ways.
Second, even up to level 24, 50K XP is always going to be enough to jump (at least) two levels. The writers must have known this, and the general expectation is that characters stop at level 20, there must be a reason they put it at 50K instead of just saying gain a level (unless it carried over from an older version and they weren't paying attention).
Third, drawing from the Deck is a big risk. They took it, and that player got lucky. Stripping off somewhere between 42 and 70 percent of the benefit (depending on how much XP the player had before the draw) seems unfair.
Four, you said the campaign is ending. Unless you meant that differently, that means the characters are retiring. So does it really matter what level they are at?
That said, if the campaign was continuing, I might negotiate with the player. Because having one player two levels higher than the rest of the party can cause any number of issues. Jealousy, encounter balance, etc.
Yes, using Wish to wish someone dead is possible and is even presented as an example of a "custom wish" apart from the bulleted suggestions.
However, it is specifically listed as an example where the Wish might not work as the caster intends. Terms of fullfillment of wishes are up to the DM - the spell description states that wishing that a villain was dead might simply time-warp one to future after the villain's passing. The DM can also come up with other ways to make the target dead without effectively killing them, like turning them into a nonliving but equally powerful creature.
So, while the target actually dying is an outcome the DM can give, Wishing something was dead is not a reliable way to kill it.
Best Answer
Well there's no base rule on that, but there are several issues with the idea, mainly with how the DM decides the outcome of your wish.
If player tried to have their character say "I wish to be higher level", the DM would very likely immediately go "hold on, your character is not aware that they are in an RPG game with mechanics like that, you can't have them say that", or if they're really mean they'll just take that into consideration when they decide how they want the wish to come out. You might just end up several "building levels" up in the air and fall flat on your face.
So keeping to realistic ways your character would wish to become more powerful, you'd have to say something like "I want to be more powerful" or "I want to be stronger" which might give you better gear or increase your stats, but not your level.
Basically put, it's practically impossible to realistically word a wish in a way that will force the DM to only interpret it as giving you experience points.