Best advice is to invest in a consult with an experience immigration attorney. The attorney is going to ask your friend several questions to determine whether she can apply for her legal permanent residence here in the United States or she has to apply for what is called an Immigrant Visa from Mexico which will eventually get her legal permanent residence status.
She will need to find what is called a VOLAG, which is a volunteer/not for profit organization that works on grants, reviews the economic needs of those who request their services before considering their case, and has a supervising attorney. It is unclear whether she has any relief available even if she is married to a U. S. Citizen. More information is needed and someone has to take the time to investigate. Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.
Your friend will need to speak with an attorney to determine if she is eligible to adjust her status in the US, or if she must apply for an immigrant visa through the US Embassy in Mexico. If the latter, she will need a waiver of the 10-year bar for "unlawful presence," assuming she has been in the US for more than one year without legal status, which can only be approved if she can establish that her US Citizen husband would suffer "extreme hardship" if she is not readmitted to the US. Since March 2013, it is possible to apply for this waiver in advance while still in the US for "immediate relatives" of US Citizens/
She needs to speak to a qualified immigration attorney. If she cannot afford one, there are not-for-profit legal clinics and legal aid centers that may be able to assist. Good luck to her on this.
The answer above is for informational purposes only and is not intended to provide legal advice or form an attorney-client relationship. The above response should not be relied upon for any specific situation. If you need legal advice specific to your needs, you should consult with a licensed attorney.