Since you said you wanted to carry your friend through it, I assume you want them to get the achievements?
If it’s more about helping with the experience or getting practice at it, you could try the Arcade Co-op Campaign maps. You won’t get achievements and the enemy waves are setup to be harder than the equivalent on regular campaign. (So Hard on those arcade maps is harder than the regular campaign-Hard, or Brutal.) But it provides multiple bases (3) so up to 3 players can play it together.