How does the help system work in munchkin?
Yes, the rules do specifically say


  Combat strength is the total of Level plus all modifiers - positive or negative - given by items and other cards.


So the helping player adds all their bonuses to yours. Remember also to count situational modifiers that are brought about by the new helper eg. a monster who is +4 against Elves would gain a +4 bonus if the person you brought in to help was an Elf but you weren't.

Just remember that Munchkin is a competitive game and it is up to you whether you assist or not (unless a specific card says otherwise). It isn't always in your (or your fellow player's) best interests to agree to help, thus forcing the player in the combat to fight alone or run away.