It depends on how you play. I would suggest you to sample some of the creatures there and find out if you can effectively handle them on your own without too much of a struggle. If you can, then you're ready. If not, then you should probably go back and do a few side quests.
Kind of, I went in at level 29, but trust me when I say it's too hard. Get up to 30-35.
Side quests are important, if your friend is level 26 and you are also level 26, leveling up to let's say 30 is an amazing thing. Why? Because those four extra levels will always be there. When you are level 40, your friend will be 36 (if she/he doesn't do any sidequests), when you are level 90 your friend will be level 86 (if she/he doesn't do any sidequests)