Is she subject to the two year foreign residence requirement? She would need to get that waived to stay in the U.S., even if she marries a citizen.

You can marry even if her visa has expired, but check to be sure that you'll be able to get a marriage license in the state where you intend to marry if she's out of status. Also, keep in mind that she needs to stay in the U.S. while her adjustment of status is pending, unless she gets advance parole permitting travel prior to its approval.