I ass-u-me that she filed the I-751 for Removal of Conditions?
Until the appointment, she should have an extension notice extending her Green Card expiration for one year. Have her keep a photocopy of the extension notice with her expired Green card. With those and her un-expired passport, she can travel outside the country and return.
After they approve the I-751, she will usually be instructed to make an appointment to appear at the local office. They should take her old card and photos and stamp her passport with an I-551 stamp, which is her temporary Green Card. The stamp should be good for one year. If she still doesn’t have her new card after a year, make an InfoPass appointment for your local CIS office and they can stamp her passport with another extension.
My wife waited over two years after her I-751 approval and never did receive her new Green Card. When she was naturalized she no longer needed it anyway.