I friend of my was trying to boost me in Scarlet Monestary today, and first run went very good through Library to get the key and then on to Cathedral where everything went fine, so we ran out and resetted the dungeon but now when i try to enter SM Cathedral im getting teleport to Orgrimmar at the Inn, whats this magic?
That's a weirdish bug. Try inviting your friend again and entering the dungeon again with him. After that, don't reset it. Fiddle with such combinations (enter alone, enter with him, reset, don't reset, wait for daily default reset etc.) until it works.
Deleting cache folder inside your WoW folder also might help.