How can I bring or rush a friend into Nightmare mode?

I have a friend up to Normal Act 3 in his own game, but I'd like to bring him into Nightmare Act 1 with me. He says he wouldn't be able to join my Nightmare party, though we haven't tried this yet. (I would suppose this extends for all inter-difficulty parties.) Is he right?

Does he need to play through certain events to join my Nightmare party, or is it possible to rush him through Diablo (last Act 4 quest) to unlock Nightmare for him? Does it depend on level restrictions, as well? Is there a simple workaround? What do I need to do to push him into Nightmare?

Actually, you may still skip entire acts. I am not sure what the update did in 1.0.2b regarding "...no longer skip entire acts in multiplayer", but nothing stops you from joining a friend's game as long as the game you are joining was created in a difficulty level that you have already unlocked. To date the latest patch is 1.0.5b. To get your friend to nightmare your friend needs to reach level 25. To do this you will just need a little gold to gear up the low level character in the Auction House.

"Rushing" someone in Diablo III is a bit different than it was in Diablo II. I have not found a quick way to accomplish this, but I have managed to shave quite a bit of time off of the process. The process I have been using involves locating the best places to gather experience and skipping mostly all of the quests. Please comment on this post if you can provide further insight.

The places I go to farm experience are:

• A1, The Imprisoned Angel - The Cursed Hold
• A2, A Royal Audience
• A3, Heart of Sin
• A4, Beneath the Spire

When the character being rushed begins gathering experience at any given place the blue items dropping on the ground should be too high level for the char to use. When the char reaches the level where he or she can actually use the blue items which are dropping that is a pretty good indicator that it is time to stop, gear up at the auction house and move to the next act. Another indicator is when the char stops getting good experience.

Auction House Tip - Make sure to get socketed items whenever possible. Also, get items with the character's prime stat, vitality, and + experience. Try and maximize the DPS for the weapons as well. Put rubies in the weapons to increase damage, and dex/prime stat gems in everything else.

Here is a step by step set of more detailed instructions:

1. First take a level one char and bring them to about level 5 by playing around in A1.
2. When the character gets between level 5 and 10, go to the Auction House and purchase items that have + experience on kill, the character's primary stat and vitality. Make sure to get items with sockets when available. Make sure to get the highest DPS weapon you can find. Place vitality and primary stat gems in any socketed items.
3. Start A1 quest, "The Imprisoned Angel, The Cursed Hold". Let the low level character join the game. Run this quest until the low level character stops gaining at least a level every run.
4. Start A2 quest, "A Royal Audience". This is one of the best places in the game for levelling. Have the lower level character join. Kill all the snakes in Emperor Hakan's anteroom, then continue killing snakes all the way to the sewers. Try not to destroy the constructs until they have spit out all of the snakes they are going to spit out. When you reach the entrance to the sewers leave the game, change the quest back to "A Royal Audience", and repeat until the low level character is level 18-19.
5. Now go to the auction house to gear up the low level char again. Make sure to get + experience items and socketed items when possible.
6. Next start the A3 quest, "Heart of Sin". Let the low level character join and run through this quest line to Azmodan until the low level char reaches 22-23.
7. It's up to you as to whether or not you make another stop at the Auction house now. Depending on the items you have found you may not need to. Start an Act 4 game, any quest. I like to run from "Beneath the Spire" on, but it's really just a matter of preference. Play A4 and kill Diablo. Keep playing A4 until you reach level 25.

Now then, from here on out:

1. Auction House
2. A1, The Imprisoned Angel - The Cursed Hold
3. Auction House
4. A2, A Royal Audience
5. Auction House
6. A3, Heart of Sin
7. Auction House
8. A4, Beneath the Spire

...rinse, repeat.

Players cannot join higher difficulties if they have not beaten the previous difficulty on their character.

You have 2 options:

1. You will need to help your friend through Normal in his play through.
2. In your case, you could start on the final quest of the Act he is on and run until you are done the first quest, once the second quest triggers for him, quit the game, you start a game where you are on the last quest of the same act, finish the act with him and he should be able to start the next act with you. This is a lot more time consuming, but is due to the patch being released that disallows players to skip entire acts of the game.