Rick and Morty (Portal gun and others) Question

[rjbri]

I don't think I was clear. The card is actually Particle Beam Wristwatch but a few pieces of equipment have this ability.
The text says:
"You may discard this card to turn any monster into a level 1 snake worth 1 level, one Treasure and no Bad Stuff. Cannot be used to gain the winning level."
My opponent is lvl9 and that opponent is fighting a lvl 12 creature. If my opponent wins the combat they will go up a level for defeating the creature and win the game.
I want to discard this piece of equipment during their combat to change the lvl 12 creature my opponent is fighting into a level 1 snake.
I have no problem with my opponent beating the snake and getting a treasure, the hang up is the language "Cannot be used to gain the winning level".
My argument is that the creatures are swapped, the opponent beats the snake, gets a treasure, but the level becomes meaningless to them because they cannot advance to level 10 and win.
My opponent's argument is that I cannot make the move at all because it says "Cannot be used to gain the winning level." and because they are about to get the winning level, the card cannot be used. It would deny them the level they are entitled to for beating the snake according to the language and thereby make it an illegal play.
Can the card be played to (in effect) allow the opponent's new snake creature to be defeated, granting the opponent a treasure, (normally grant 1 level) but deny the opponent from gaining that level because it would be the winning level? Or is the move illegal all together as my opponent states?