You need to have the quest 7. Heart of Sin - Kill Azmodan, in Normal, Nightmare or Hell.
1) YOU start the quest, run to the boss portal and kill Azmodan, now invite YOUR FRIEND.
2) When YOUR FRIEND joins your game, he teleports to you using the banner in the city and YOU leave your game. (Make sure he is in the boss room before you leave, or he wont be able to complete the quest)
3) When YOU are out, YOUR FRIEND clicks the orb and gets XP (1), he goes back to town, talks to the guard, goes to the armory and enters the portal, he will get the second part of the quest XP (2)
4) Now YOU click Resume Game, and invite YOUR FRIEND. Now you'll both be at the Azmodan room, this time YOU dont leave the game yet, just click the orb and you will both complete the quest and he will get XP again (3).
5) After he gets xp, YOU leave the game again, and YOUR FRIEND waits until you are out and completes the second part of the quest, he'll get XP again (4)
1) YOU click Resume Game one more time, invite YOUR FRIEND and now you'll both be at the city. You both complete the second part of the quest and he gets XP (5)
This way your friend can complete 5 quests instead of the two. I've been power leveling my friends in Hell Mode and they're getting around 90k XP per quest (~450k xp in 5minutes)
If you leave the game when you're not supposed to, your friend's game will disconnect, if you follow the guide he will complete 5 quests and level very fast