There are a lot of stories of LPRs coming back after extended absences. She has a valid GC so she should be able to board a flight to the U.S. She may face scrutiny at CBP, but a CBP officer cannot take away her LPR status. If you are concerned about getting in, I would plan to accompany her at least on the flight that enters the U.S., but otherwise, buy a ticket and have her come back.
Good Luck!