The justice system treats you, as the "lookout", just as important to the commission of the crime as the person who committed it. However, it does not sound like you did anything wrong - just being there does not make one guilty. You should have fought the case (and got your "friend" to admit that you told her you wanted nothing to do with her criminal acts.)
Why didn't you fight the charges after you were arrested? You make it sound as if you had not choices in how your case was handled. You always have the chance to point out to the court through your attorney that you had nothing to do with the vandalism and theft, but apparently you didn't.
As for standing there watching the whole thing go on and yet you didn't walk away from it or try to stop her. Those are things that are required if you want to avoid responsibility for those crimes. You must walk away next time or the same thing could happen. The whole point of the system is to get people to stop breaking the law and be law abiding citizens. It may seem unfair to you in this particular case, but overall, its the best system out there. Its a lot worse in other nations.
Next time, make sure to walk away or stop the other person before they start and don't share in anything taken or the same thing could happen again. Of course, if you get in any other trouble, make sure to hire a good lawyer who will assist you to fight the charges next time.
One reason which comes to mind why you may have received the same punishment as your criminal friend is because you may have admitted to the same crime, effectively convicting yourself of it. If you indeed did not know what your friend was planning to do, were totally surprised by it, and took immediate steps to stop it, you should have fought the case by asking for a trial. The jury could not have convicted you unless they were unanimously convinced beyond any reasonable doubt that you were guilty, and this is often a tough thing for the authorities to prove. There is a good chance that a jury would have found you not guilty, which would have meant no punishment at all for you in that event. Our court system only protects people to the extent that they protect themselves by fighting unfair charges at trial. Pleading guilty or no contest, on the other hand, will virtually guarantee a similar punishment to that given to your accomplice. Representation by an experienced criminal lawyer is essential for success in fighting or resolving any case.