Since you're asking this on someone else's behalf, it is very difficult to give you a good answer as there is a lot of information necessary. If she mailed a residency application, but never received anything then she is not a resident and cannot apply for citizenship yet. If that is not the case, please get more facts (when did she apply for a green card, was she approved, were conditions removed, has she applied for citizenship, is she still married to a US citizen, if yes is it the same person she got her green card through if she has it, etc. and when these things happened at a bare minimum) so you can ask a specific question and get a better answer.
Please do not confuse legal permanent residence with US citizenship. Confusing the two can result in a false claim to US citizenship. A false claim to US citizenship results in deportation and a permanent bar from the US.
You should retain an experienced immigration lawyer to review all the facts, advise you, and handle the case. You can find one through http://www.ailalawyer.com.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
She needs to file an I-90 Form to receive the green card you are saying she never received until now, most likely because she changed her address and never properly notified USCIS as she was supposed to.
I suggest that before filing the I-90 she schedule an INFOPASS appointment at her local USCIS district office to understand what really took place.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.