It sounds as if she is a Conditional LPR. If so, she will have to file her I-751 within 90 days of her card expiring. This is a difficult situation, because it seems as if her spouse is no where to be found and he would not be signing the form (and under no circumstance should she forge his signature), I recommend she consult with a local attorney on this case, BEFORE, her card expires. USCIS has published several Memo's speaking to filing when an ex-spouse is no where to be found. Ultimately, she will have to prove this marriage was in good faith, or one of the other exceptions.
It sounds like your friend needs to file the Petition to Remove Conditions of Residence. This two year condition is placed when a person obtains their lawful permanent residence through their spouse. It is generally necessary for the couple to make the petition together, but it is possible to petition on her own, but more challenging. She has to prove that the marriage was not for immigration purposes. She should consult with an attorney to assess the particular facts of her case.
The other two attorneys are correct: the whereabouts of her I-864 'financial' sponsor isn't as important as the whereabouts of her husband ... that will control how the forms are done. There are low-cost attorneys available, see below.
Franco Capriotti - Senior Legal Counsel & Former Professor of Immigration Law - Capriotti International Law - firstname.lastname@example.org
Our services are not limited by State, nor International bounderies. Our licenses allow us to represent people worldwide.
Do not rely completely on information you get on any website. It is always wise to consult personally with an immigration attorney before taking action.
Consider contacting one of the attorneys on Avvo, or www.ailalawyer.com - If you are low income go here: www.justice.gov/eoir/probono/states.htm or http://www.immigrationlawhelp.org/
Capriotti International Law
FORMER IMMIGRATION LAW PROFESSOR -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.