When are you allowed to play monster boosters in Munchkin Quest?
I agree with Pat I think, but it was a different section of the rules that caught my eye:


  Certain cards, called monster
  enhancers, raise or lower the combat
  strength of individual monsters. (Yes,
  you can have a negative enhancement.)
  Monster enhancers may be played by any
  player during any combat.


I don't know if we're looking at the same version of the rules or different ones, but the general facts of the case seem clear!