In Munchkin, can someone who doesn't participate in a combat play cards that benefit a player who is fighting a monster?
You can absolutely play cards to help other players, even when not involved in combat

From the rules (direct PDF link)


  You can interfere with others’ combats in several ways, including: 
  
  Use a one-shot card. You could help another player by using a
  one-shot to strengthen his side. Of course, you can “accidentally”
  strengthen the monster with it, instead . . .


Emphasis mine