Boss monster spell cards
He probably shouldn't have been able to play the spell.

The game doesn't have comprehensive rules on the order of announcing spells. However, there's no "stack". The stack is a feature of Magic and nobody should assume that Magic's reactionary play and LIFO resolution system applies to other games without them saying so. The majority of games (especially light games like this) work on the assumption that stuff happens as soon as you play it, unless something else explicitly says it can be played in response.

The closest the rules get to this is the following (page 5):


  When you’re the “active player” (when you’re building a Room, or Heroes are moving through your dungeon) your Spell effects are resolved first. Other players’ Spells and effects are resolved in XP order.


Essentially, everyone piles in with their desire to cast a spell and then you see how they resolve. This makes it clear that the designers understand more than one person can be casting at once, and how to resolve it (i.e. not a stack).

In the situation above, the destruction of the room already occurred, causing the discard. Unless the spell is specifically worded to be played in reaction to that situation, I'd recommend just resolving things as they happen.