She must first be a permanent resident. If she has been a permanent resident for at least three years then she can apply for citizenship. If she is not a permanent resident then her husband must file an I-130 visa petition. The best thing to do is have your friend and his wife meet with an immigration attorney for a consultation.
The answer provided here is general in nature and does not take into account other factors that may need to be reviewed for a more precise answer. You should consult with an immigration attorney before taking any action. The answer here is not intended to create an attorney-client relationship.
Based on your additional information it appears they need to first adjust her status to that of lawful permanent resident. That is the first step in the process. Using a visa to enter is good news as that likely makes her eligible pursuant to INA 245(a).