Hello
I am playing multiplayer co-op with my friend and we are completing some quests together. As we came to the quest called "the eye of hell" we both went down the teleport to "hell". When we were done slaughtering all the monsters and received the key/rune to get up from the cave, it was only my friend who could get up.
So now I cannot get up from the cave and really don’t know what to do.
I really hope there is someone who can help me?

If your friend made it out, you can teleport to his location in multiplayer. IIRC, use either the control key or the shift key and mouse click.