Your friend will be required to make a showing of factual innocence to get the arrest removed from her record. Factual innocence is hard to show, and there are some serious downsides to attempting show factual innocence if the statute of limitations has not expired.
As a side note, in 2009 CA started taking DNA samples of people arrested -- not convicted -- for felony crimes, and she can petition the court to have her DNA expunged from the Department of Justice DNA database without a showing of factual innocence. I wouldn't want the government to have my DNA, and that's something worth mentioning to her.
Yes I would recommend hiring an attorney. Also, just because charges have not been filed yet does not mean that charges will not be filed. This is not an easy process. For more information review California Penal Code 851.8 PC
Mr. Driessen is a former Deputy DA in Orange County with over 8 years of criminal law experience. Nothing stated on this site shall in anyway be construed as legal advice, or as creating any attorney client relationship. If you would like to hire Mr. Driessen, feel free to contact him at www.theocduiguy.com.
My understanding is you can only get an arrest removed from your record if you are factually innocent. I don't know that you would be able to get it removed while the staute of limitations is still running.