So I just read this on d2jsp and thought I'd share.
So if you're attempting say... Belial Hardcore and need an escape back up plan.
1. Give your friend the way point (after saving the people).
2. Have him create a new game by resuming.
3. Join his game and continue to Belial (you shouldn't get any more check points until you kill Belial)
4. Have your friend be prepared to hit exit game.
5. Attempt Belial and if you're hitting enrage timer, simply have your friend leave the game and you will also instantly disconnect with the new changes.
This can obviously be applied to other situations throughout the game as long as you don't finish any quests or obtain a new checkpoint. Obviously Belial is just an example since you would be fighting buffed monsters with 2 people but say if you were attempting to run to the next checkpoint and skipping elite packs, this can be used.