As I understand the sequence of events:
- Player A is fighting the Potted Plant.
- Player X (you) decide to make the combat harder, and reanimate the Plutonium Dragon as Undead, adding it to the fight.
- Player A asks Player B to help, and B agrees.
- Player B plays Lily Pollen, making the (Undead) Plutonium Dragon go away.
- Player X (you) protests, because you intended to play the Cytellish Potion to force player B to wander off and player A to fight the monsters alone.
Assuming that I have the above sequence correct, there was no misplay. The ONLY time the munchkin fighting a monster is required to give the other players a reasonable time to respond is when he declares that he is winning the fight. There is nothing wrong with Player B joining the fight and immediately playing his
Lily Pollen card. (There would have been nothing wrong with Player B playing his card without joining the fight, in fact, but I can see where he was using it as leverage.)
If you played your
Cytellish Potion anyway, it would not reverse any of the events of the combat up until then, except that Player B would no longer be helping, and if Player A won he would no longer be required to reveal his Treasures.