I found that its pretty quick just to load a game and do any of the quests for hell rift in act 4 and also do the act 1 scavengers den or khazra and just repeat. You make the game on highest difficulty that you can kill stuff on and just let your friend sit there and get the quest completion.
There is also the act 1 Mothers thing but it's really only good at the extreme lower levels, I went from one tosix orseven in one kill, and I didn't have to kill it, the game did. But I find that extremely boring and you end up penalizing yourself as you'll have no gear when you get leveled up. Better to work up through it.