Did you do the crime? If so, you certainly can contact the prosecuting attorney and admit to this. Be cautioned that if your girlfriend did commit the crime and/or she was an accomplice, you could end up yourself in trouble and not do a thing to assist her. She should speak to her attorney and obtain discovery from the prosecutor. If you are implicated in any way, it will likely be in the discovery.
I admire your nobility here sir, but be very careful and consult with an attorney and gather the facts before you offer yourself up as a sacrificial lamb.
This answer is marginal legal advice and does not constitute an attorney-client relationship. Every client and case is unique. The best advice is to always consult with an attorney. Free legal resources at www.ZippToCourt.com
Very, very dangerous. The most likely outcome is that nobody would believe your statement and in the end she would do time for theft and you would do additional time for perjury and obstruction of justice. Bad idea. Very bad idea.
Whatever you do, do not go to the State's Attorney or police without a lawyer. If you are in Chicago, you can call First Defense Legal Aid at 1-800-LAW-REP-4. FDLA provides free representation to those under investigation or arrest by the Chicago Police Department. If your're not in Chicago, call a lawyer in your area (or 2 or 3!) for a free consultation.
This answer should not be considered legal advice, is for informational purposes only, and no attorney-client relationship should be assumed. Most lawyers offer free consultations and you should not hesitate to seek out a lawyer to assist you with this matter.