Does "-5 on your next combat" include helping someone?
The Official Munchkin FAQ states:


  Q. If a card that affects "your next combat" is played on you during a combat, does it affect THAT combat, or the next one you're in?
  A. If the combat is still unresolved (and obviously it is, because people are still playing cards on you for it), then that combat is the "next" one. Most Munchkin games say this explicitly in the rules, but it's true even if we left it out by mistake.


That seems quite clear. The next combat to resolve  in which "you are a participant".