You're not talking about citizenship, you're referring to adjusting status ("getting a green card"). Citizenship is later down the road. Folks entering on the Visa Waiver Program are subject to different rules than other entrants into the U.S. They can be approved for an adjustment of status, but only if certain conditions are met and the case is properly prepared. Definitely touch base with an experienced immigration attorney on this and they can advise you as to preconceived intent issues as well, depending on how soon your friend married after she/he entered.
If she files the application for permanent residence (not citizenship) before her 90 days expire, then she can stay in the U.S. until there is a decision on her application. At the interview the immigration officer will decide whether to approve or deny her application. If the officer believes that she lied to enter the U.S. as a visitor (when all along she intended to stay, get married and apply for a green card) then her application will be denied. It would be wise of your friend to consult with an immigration lawyer before filing anything.
She is not eligible to file for citizenship until she has been a permanent resident for three years. Meanwhile, she may stay in the U.S. while her adjustment application is pending, assuming she filed that.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
I agree with my colleagues. It's a three step process to citizenship by naturalization. The first step is seeking conditional resident status based upon a marriage to a U. S. Citizen that is entered into based upon a good faith relationship and mutual moral respect.
This is general information, not legal advice, and does not create an attorney client relationship.