Question about Adventure Time card

[KrodMandoon]

My buddies were playing the Adventure Time version of Munchkin and came running to me for an unbiased ruling on Prismos Pickles. I haven't played that version yet, but they brought me Prismos Pickles and wanted me to pay special attention to the "AS SOON" part of it. The player who kicked open the door turned a monster, read the monster card to the other players (only their 2nd time playing so cards were being read aloud to ensure everyone knew what they did), and then declared he wanted to kill it for the winning level. As he declared, one of the other guys dropped Prismos Pickles. After reading the card, I said that it seemed like a legit play since nothing was done after the door being kicked except for the reading of the card.

So was that played legally? Should the player have used it AS SOON as the card was flipped over before knowing what it was?