Question on usage of Transferral Potion

[Zanatar]

Ok I was playing a game of Munchkin last night in which my son was in a scenario that won him the game. I want to make sure what he did is in fact a legal move in relation to this card.

Basically he was level 8. I, myself was fighting a monster (King Tut) and I could not beat it. My son offered to help me kill the monster , in exchange of having 2 of the 4 treasures. I accepted the offer.

He then played the "Transferral Potion" card, in which he stole the monster I was fighting form me, and he fought it himself. He was able to kill the monster , which granted him all 4 treasures (which is moot) and 2 levels, which won him the game, lvl 8 to 10.

Is that a legal use of that "Transferral Potion" card? Given the way he used it?

If he did , I applaud him, as that is very much in the nature and spirit of the game.

Thanks,
Zanatar.